cmd/compile: wrong calculation result for bit operation that's inlined and has all constant shifts in rewrite rules [1.12 backport]

[gopherbot]

@dr2chase requested issue #32680 to be considered for backport to the next 1.12 minor release.

@gopherbot please open backport issues. It technically fails the regression policy (been broken too long -- probably since 1.9) but it is a silent bad code problem which is very bad, and the fix is very low risk.

[dr2chase]

Just in case -- a manual step is mandatory,

  cd cmd/compile/internal/ssa/gen
  go run *.go

[gopherbot]

Change https://golang.org/cl/183241 mentions this issue: [release-branch.go1.12] cmd/compile: add necessary operand to mergePoint in rewrite rules

[gopherbot]

Closed by merging 4dbe72d to release-branch.go1.12.