cmd/compile: reduce bounds checks in generated rewrite rules

cmd/compile: reduce bounds checks in generated rewrite rules

CL 213703 converted generated rewrite rules for commutative ops
to use loops instead of duplicated code.

However, it loaded args using expressions like
v.Args[i] and v.Args[i^1], which the compiler could
not eliminate bounds for (including with all outstanding
prove CLs).

Also, given a series of separate rewrite rules for the same op,
we generated bounds checks for every rewrite rule, even though
we were repeatedly loading the same set of args.

This change reduces both sets of bounds checks.

Instead of loading v.Args[i] and v.Args[i^1] for commutative loops,
we now preload v.Args[0] and v.Args[1] into local variables,
and then swap them (as needed) in the commutative loop post statement.

And we now load all top level v.Args into local variables
at the beginning of every rewrite rule function.

The second optimization is the more significant,
but the first helps a little, and they play together
nicely from the perspective of generating the code.

This does increase register pressure, but the reduced bounds
checks more than compensate.

Note that the vast majority of rewrite rules evaluated
are not applied, so the prologue is the most important
part of the rewrite rules.

There is one subtle aspect to the new generated code.
Because the top level v.Args are shared across rewrite rules,
and rule evaluation can swap v_0 and v_1, v_0 and v_1
can end up being swapped from one rule to the next.
That is OK, because any time a rule does not get applied,
they will have been swapped exactly twice.

Passes toolstash-check -all.

name        old time/op       new time/op       delta
Template          213ms ± 2%        211ms ± 2%  -0.85%  (p=0.000 n=92+96)
Unicode          83.5ms ± 2%       83.2ms ± 2%  -0.41%  (p=0.004 n=95+90)
GoTypes           737ms ± 2%        733ms ± 2%  -0.51%  (p=0.000 n=91+94)
Compiler          3.45s ± 2%        3.43s ± 2%  -0.44%  (p=0.000 n=99+100)
SSA               8.54s ± 1%        8.32s ± 2%  -2.56%  (p=0.000 n=96+99)
Flate             136ms ± 2%        135ms ± 1%  -0.47%  (p=0.000 n=96+96)
GoParser          169ms ± 1%        168ms ± 1%  -0.33%  (p=0.000 n=96+93)
Reflect           456ms ± 3%        455ms ± 3%    ~     (p=0.261 n=95+94)
Tar               186ms ± 2%        185ms ± 2%  -0.48%  (p=0.000 n=94+95)
XML               251ms ± 1%        250ms ± 1%  -0.51%  (p=0.000 n=91+94)
[Geo mean]        424ms             421ms       -0.68%

name        old user-time/op  new user-time/op  delta
Template          275ms ± 1%        274ms ± 2%  -0.55%  (p=0.000 n=95+98)
Unicode           118ms ± 4%        118ms ± 4%    ~     (p=0.642 n=98+90)
GoTypes           983ms ± 1%        980ms ± 1%  -0.30%  (p=0.000 n=93+93)
Compiler          4.56s ± 6%        4.52s ± 6%  -0.72%  (p=0.003 n=100+100)
SSA               11.4s ± 1%        11.1s ± 1%  -2.50%  (p=0.000 n=96+97)
Flate             168ms ± 1%        167ms ± 1%  -0.49%  (p=0.000 n=92+92)
GoParser          204ms ± 1%        204ms ± 2%  -0.27%  (p=0.003 n=99+96)
Reflect           599ms ± 2%        598ms ± 2%    ~     (p=0.116 n=95+92)
Tar               227ms ± 2%        225ms ± 2%  -0.57%  (p=0.000 n=95+98)
XML               313ms ± 2%        312ms ± 1%  -0.37%  (p=0.000 n=89+95)
[Geo mean]        547ms             544ms       -0.61%

file    before    after     Δ       %
compile 21113112  21109016  -4096   -0.019%
total   131704940 131700844 -4096   -0.003%

Change-Id: Id6c39e0367e597c0c75b8a4b1eb14cc3cbd11956
Reviewed-on: https://go-review.googlesource.com/c/go/+/216218
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>