[release-branch.go1.14] cmd/compile: mark s390x int <-> float conversions as clobbering flags

These conversion instructions set the condition code and so should
be marked as clobbering flags.

Updates #39651.
Fixes #39690.

Change-Id: I1e3f2cf33337128d321b52ac72f46d1b8798ebd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/242237


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>