cmd/compile: unexpected nil dereference on s390x [1.13 backport]

[gopherbot]

@mundaym requested issue #38195 to be considered for backport to the next 1.13 minor release.

@gopherbot please consider this for a backport

This is a bug in the register allocator that results in the compiled program behaving incorrectly. There is currently no workaround and it results in very hard to debug issues. It has only been seen to cause problems on s390x but theoretically it affects all platforms.

[gopherbot]

Change https://golang.org/cl/230358 mentions this issue: [release-branch.go1.13] cmd/compile: fix deallocation of live value copies in regalloc

[gopherbot]

Closed by merging 6b974e2 to release-branch.go1.13.