cmd/compile: merge more shifts into stores

Updates #36223

(Might fix #36223. I'm not sure whether there are more outstanding.)

This helps a bit, but not as much as I'd expected/hoped.

file                                      before  after   Δ       %       
runtime.s                                 477286  477256  -30     -0.006% 
bytes.s                                   31089   31085   -4      -0.013% 
time.s                                    83561   83547   -14     -0.017% 
strings.s                                 43284   43280   -4      -0.009% 
compress/flate.s                          51374   51295   -79     -0.154% 
math/big.s                                184283  184256  -27     -0.015% 
crypto/elliptic.s                         51649   51577   -72     -0.139% 
crypto/sha512.s                           8661    8644    -17     -0.196% 
crypto/sha1.s                             6975    6959    -16     -0.229% 
crypto/sha256.s                           6412    6393    -19     -0.296% 
vendor/golang.org/x/text/unicode/bidi.s   27158   27146   -12     -0.044% 
vendor/golang.org/x/text/unicode/norm.s   66802   66788   -14     -0.021% 
net/http.s                                560936  560929  -7      -0.001% 
text/template.s                           96475   96467   -8      -0.008% 
go/parser.s                               80284   80280   -4      -0.005% 
text/tabwriter.s                          9618    9611    -7      -0.073% 
go/printer.s                              78502   78499   -3      -0.004% 
go/types.s                                321815  321807  -8      -0.002% 
internal/xcoff.s                          23175   23171   -4      -0.017% 
image/jpeg.s                              36609   36587   -22     -0.060% 
cmd/vendor/golang.org/x/arch/x86/x86asm.s 81274   81001   -273    -0.336% 
cmd/internal/obj.s                        115184  115126  -58     -0.050% 
cmd/internal/obj/arm64.s                  151502  151487  -15     -0.010% 
cmd/internal/obj/s390x.s                  128054  128046  -8      -0.006% 
cmd/internal/obj/wasm.s                   44295   44291   -4      -0.009% 
cmd/compile/internal/ssa.s                4201992 4209504 +7512   +0.179% 
cmd/compile/internal/gc.s                 1555029 1555011 -18     -0.001% 
total                                     9792875 9799640 +6765   +0.069% 


Change-Id: If4a857c0953a766578e68aa299b112a20d9b2b86
Reviewed-on: https://go-review.googlesource.com/c/go/+/213704


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>