cmd/compile: ensure temp register mask isn't empty

cmd/compile: ensure temp register mask isn't empty

We need to avoid nospill registers at this point in regalloc.
Make sure that we don't restrict our register set to avoid registers
desired by other instructions, if the resulting set includes only
nospill registers.

Fixes #57846

Change-Id: I05478e4513c484755dc2e8621d73dac868e45a27
Reviewed-on: https://go-review.googlesource.com/c/go/+/461685
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>