cmd/compile: invalid pointer found on stack when compiled with -race [1.21 backport]

[gopherbot]

@randall77 requested issue #63657 to be considered for backport to the next 1.21 minor release.

@gopherbot Please open a backport issue for 1.21.

[randall77]

This is a rare but serious miscompilation, causing invalid pointers to be seen by the garbage collector.
(It can happen without -race.)

[gopherbot]

Change https://go.dev/cl/538717 mentions this issue: cmd/compile: ensure pointer arithmetic happens after the nil check

[gopherbot]

Change https://go.dev/cl/538857 mentions this issue: cmd/compile: handle constant pointer offsets in dead store elimination

[gopherbot]

Closed by merging caacf3a to release-branch.go1.21.