cmd/compile: tighten after lowering

cmd/compile: tighten after lowering

Moving tighten after lowering benefits from the removal of values by
lowering and lowered CSE. It lets us make better decisions about
which values are rematerializable and which generate flags.
Empirically, it lowers stack usage (by avoiding spills)
and generates slightly smaller and faster binaries.

Fixes #19853
Fixes #21041

name        old time/op       new time/op       delta
Template          195ms ± 4%        193ms ± 4%  -1.33%  (p=0.000 n=92+97)
Unicode          94.1ms ± 9%       92.5ms ± 8%  -1.66%  (p=0.002 n=97+95)
GoTypes           572ms ± 5%        566ms ± 7%  -0.92%  (p=0.001 n=95+98)
Compiler          2.56s ± 4%        2.52s ± 3%  -1.41%  (p=0.000 n=94+97)
SSA               6.52s ± 2%        6.47s ± 3%  -0.82%  (p=0.000 n=96+94)
Flate             117ms ± 5%        116ms ± 7%  -0.72%  (p=0.018 n=97+97)
GoParser          148ms ± 6%        146ms ± 4%  -0.97%  (p=0.002 n=98+95)
Reflect           370ms ± 7%        363ms ± 6%  -1.79%  (p=0.000 n=99+98)
Tar               175ms ± 6%        173ms ± 6%  -1.11%  (p=0.001 n=94+95)
XML               204ms ± 6%        201ms ± 5%  -1.49%  (p=0.000 n=97+96)
[Geo mean]        363ms             359ms       -1.22%

name        old user-time/op  new user-time/op  delta
Template          251ms ± 5%        245ms ± 5%  -2.40%  (p=0.000 n=97+93)
Unicode           131ms ±10%        128ms ± 9%  -1.93%  (p=0.001 n=100+99)
GoTypes           760ms ± 4%        752ms ± 4%  -0.96%  (p=0.000 n=97+95)
Compiler          3.51s ± 3%        3.48s ± 2%  -1.04%  (p=0.000 n=96+95)
SSA               9.57s ± 4%        9.52s ± 2%  -0.50%  (p=0.004 n=97+96)
Flate             149ms ± 6%        147ms ± 6%  -1.46%  (p=0.000 n=98+96)
GoParser          184ms ± 5%        181ms ± 7%  -1.84%  (p=0.000 n=98+97)
Reflect           469ms ± 6%        461ms ± 6%  -1.69%  (p=0.000 n=100+98)
Tar               219ms ± 8%        217ms ± 7%  -0.90%  (p=0.035 n=96+96)
XML               255ms ± 5%        251ms ± 6%  -1.48%  (p=0.000 n=98+98)
[Geo mean]        476ms             469ms       -1.42%

name        old alloc/op      new alloc/op      delta
Template         37.8MB ± 0%       37.8MB ± 0%  -0.17%  (p=0.000 n=100+100)
Unicode          28.8MB ± 0%       28.8MB ± 0%  -0.02%  (p=0.000 n=100+95)
GoTypes           112MB ± 0%        112MB ± 0%  -0.20%  (p=0.000 n=100+97)
Compiler          466MB ± 0%        464MB ± 0%  -0.27%  (p=0.000 n=100+100)
SSA              1.49GB ± 0%       1.49GB ± 0%  -0.08%  (p=0.000 n=100+99)
Flate            24.4MB ± 0%       24.3MB ± 0%  -0.25%  (p=0.000 n=98+99)
GoParser         30.7MB ± 0%       30.6MB ± 0%  -0.26%  (p=0.000 n=99+100)
Reflect          76.4MB ± 0%       76.4MB ± 0%    ~     (p=0.253 n=100+100)
Tar              38.9MB ± 0%       38.8MB ± 0%  -0.20%  (p=0.000 n=100+97)
XML              41.5MB ± 0%       41.4MB ± 0%  -0.19%  (p=0.000 n=100+98)
[Geo mean]       77.5MB            77.4MB       -0.16%

name        old allocs/op     new allocs/op     delta
Template           381k ± 0%         381k ± 0%  -0.15%  (p=0.000 n=100+100)
Unicode            342k ± 0%         342k ± 0%  -0.01%  (p=0.000 n=100+98)
GoTypes           1.19M ± 0%        1.18M ± 0%  -0.24%  (p=0.000 n=100+100)
Compiler          4.52M ± 0%        4.50M ± 0%  -0.29%  (p=0.000 n=100+100)
SSA               12.3M ± 0%        12.3M ± 0%  -0.11%  (p=0.000 n=100+100)
Flate              234k ± 0%         234k ± 0%  -0.26%  (p=0.000 n=99+96)
GoParser           318k ± 0%         317k ± 0%  -0.21%  (p=0.000 n=99+100)
Reflect            974k ± 0%         974k ± 0%  -0.03%  (p=0.000 n=100+100)
Tar                392k ± 0%         391k ± 0%  -0.17%  (p=0.000 n=100+99)
XML                404k ± 0%         403k ± 0%  -0.24%  (p=0.000 n=99+99)
[Geo mean]         794k              792k       -0.17%

name        old object-bytes  new object-bytes  delta
Template          393kB ± 0%        392kB ± 0%  -0.19%  (p=0.008 n=5+5)
Unicode           207kB ± 0%        207kB ± 0%    ~     (all equal)
GoTypes          1.23MB ± 0%       1.22MB ± 0%  -0.11%  (p=0.008 n=5+5)
Compiler         4.34MB ± 0%       4.33MB ± 0%  -0.15%  (p=0.008 n=5+5)
SSA              9.85MB ± 0%       9.85MB ± 0%  -0.07%  (p=0.008 n=5+5)
Flate             235kB ± 0%        234kB ± 0%  -0.59%  (p=0.008 n=5+5)
GoParser          297kB ± 0%        296kB ± 0%  -0.22%  (p=0.008 n=5+5)
Reflect          1.03MB ± 0%       1.03MB ± 0%  -0.00%  (p=0.008 n=5+5)
Tar               332kB ± 0%        331kB ± 0%  -0.15%  (p=0.008 n=5+5)
XML               413kB ± 0%        412kB ± 0%  -0.19%  (p=0.008 n=5+5)
[Geo mean]        728kB             727kB       -0.17%

Change-Id: I9b5cdb668ed102a001897a05e833105acba220a2
Reviewed-on: https://go-review.googlesource.com/95995
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>