cmd/compile: stop MOVW-ing -1 as SRA shift amount in mips

The shift amount in SRAconst needs to be in the [0,31] range, so stop
MOVWing -1 to SRA in the Rsh lowering rules.

Also see CL 270117.

Passes

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

Updates #42587

Change-Id: Ib5eb99b82310e404cc2d6f0c619b21b8a15406ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/270558


Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>