32-bit random data corruption [1.15 backport]

[gopherbot]

@randall77 requested issue #43570 to be considered for backport to the next 1.15 minor release.

I have a small repro that also works on 64 bit.
It's been broken since 1.12.
@gopherbot please open backport issues.

[gopherbot]

Change https://golang.org/cl/282558 mentions this issue: cmd/compile: don't short-circuit copies whose source is volatile

[randall77]

This bug causes random corruption in rare cases involving large structs being copied from function result to heap to function argument. Workaround are possible but difficult.