cmd/compile: keep typecheck results in syntax tree

cmd/compile: keep typecheck results in syntax tree

Saves on both space and cost of map operations. Saves about 3% in compile time.

name                      old time/op       new time/op       delta
Template                        251ms ± 2%        244ms ± 1%   -2.78%  (p=0.000 n=8+8)
Unicode                         149ms ± 5%        135ms ± 2%   -9.03%  (p=0.000 n=10+10)
GoTypes                         1.38s ± 1%        1.35s ± 1%   -2.29%  (p=0.000 n=10+10)
Compiler                        115ms ± 2%        112ms ± 2%   -2.50%  (p=0.001 n=10+9)
SSA                             11.9s ± 0%        11.4s ± 0%   -4.04%  (p=0.000 n=9+10)
Flate                           153ms ± 1%        148ms ± 1%   -3.32%  (p=0.000 n=10+9)
GoParser                        284ms ± 2%        280ms ± 1%   -1.70%  (p=0.002 n=10+10)
Tar                             209ms ± 2%        205ms ± 2%   -1.98%  (p=0.004 n=9+10)
XML                             287ms ± 2%        281ms ± 1%   -2.06%  (p=0.000 n=10+10)
LinkCompiler                    508ms ± 2%        501ms ± 2%   -1.31%  (p=0.024 n=9+9)
ExternalLinkCompiler            2.66s ± 3%        2.63s ± 4%     ~     (p=0.280 n=10+10)
LinkWithoutDebugCompiler        338ms ± 3%        330ms ± 3%   -2.21%  (p=0.009 n=10+10)
StdCmd                          21.5s ± 1%        20.8s ± 1%   -3.27%  (p=0.000 n=9+9)
[Geo mean]                      615ms             597ms        -2.91%

name                      old user-time/op  new user-time/op  delta
Template                        344ms ± 2%        324ms ± 3%   -6.01%  (p=0.000 n=9+9)
Unicode                         215ms ±11%        192ms ± 2%  -10.84%  (p=0.000 n=10+9)
GoTypes                         1.99s ± 2%        1.93s ± 2%   -2.73%  (p=0.000 n=10+10)
Compiler                        142ms ± 4%        140ms ± 3%   -1.89%  (p=0.031 n=9+9)
SSA                             17.4s ± 1%        17.0s ± 5%     ~     (p=0.113 n=9+10)
Flate                           200ms ± 4%        196ms ± 6%     ~     (p=0.190 n=10+10)
GoParser                        388ms ± 3%        378ms ± 4%   -2.59%  (p=0.004 n=9+10)
Tar                             278ms ± 8%        277ms ± 2%     ~     (p=0.315 n=10+10)
XML                             387ms ± 2%        381ms ± 2%   -1.63%  (p=0.005 n=8+8)
LinkCompiler                    784ms ± 4%        778ms ± 2%     ~     (p=0.436 n=10+10)
ExternalLinkCompiler            2.45s ± 1%        2.42s ± 1%   -1.11%  (p=0.001 n=10+9)
LinkWithoutDebugCompiler        374ms ± 3%        366ms ± 2%   -2.15%  (p=0.010 n=10+9)
[Geo mean]                      600ms             583ms        -2.91%

Change-Id: I9552a70d6a2ad500e9acd8815762b761be3c2ff9
Reviewed-on: https://go-review.googlesource.com/c/go/+/432897
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>