[release-branch.go1.12] cmd/compile: add necessary operand to mergePoint in rewrite rules
A missing operand to mergePoint caused lower to place values
in the wrong blocks.
Includes test, belt+suspenders to do both ssa check and verify
the output (was is how the bug was originally observed).
The fixed bug here is very likely present in Go versions
1.9-1.12 on amd64 and s390x
Updates #32680.
Fixes #32712.
Change-Id: I63e702c4c40602cb795ef71b1691eb704d38ccc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/183059
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit
769fda2d5110eef6146d7df3bf0219872c9b0da6)
Reviewed-on: https://go-review.googlesource.com/c/go/+/183241