cmd/compile: mask SLL,SRL,SRAconst shift amount

cmd/compile: mask SLL,SRL,SRAconst shift amount

mips SRA/SLL/SRL shift amounts are used mod 32; this change aligns the
XXXconst rules to mask the shift amount by &31.

Passes

  $ GOARCH=mips go build -toolexec 'toolstash -cmp' -a std
  $ GOARCH=mipsle go build -toolexec 'toolstash -cmp' -a std

Fixes #42587

Change-Id: I6003ebd0bc500fba4cf6fb10254e1b557bf8c48f
Reviewed-on: https://go-review.googlesource.com/c/go/+/270117
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>