cmd/compile: s390x floating point <-> integer conversions clobbering the condition code [1.13 backport]

[gopherbot]

@mundaym requested issue #39651 to be considered for backport to the next 1.13 minor release.

@gopherbot please consider this for a backport

This is a bug that affects the control flow of a program running on s390x and therefore can result in it behaving incorrectly. The fix is simple and safe and only affects s390x.

[dmitshur]

Approved for 1.14 (#39690) and 1.13 (this issue) as it's a serious problem without a workaround.

[gopherbot]

Change https://golang.org/cl/242238 mentions this issue: [release-branch.go1.13] cmd/compile: mark s390x int <-> float conversions as clobbering flags

[dmitshur]

Go 1.13 is no longer supported per release policy, so this backport issue is no longer needed and can be closed.