cmd/compile: lower priority of avoiding registers

We avoid allocating registers when we know they may have a fixed use
later (arg/return value, or the CX shift argument to SHRQ, etc.) But
it isn't worth avoiding that register if it requires moving another
register.

A move we may have to do later is not worth a move we definitely have
to do now.

Fixes #59288

Change-Id: Ibbdcbaea9caee0c5f3e0d6956a1a084ba89757a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/479895
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/ssa/regalloc.go

@@ -1688,7 +1688,7 @@ func (s *regAllocState) regalloc(f *Func) {
 						}
 					}
 					// Avoid registers we're saving for other values.
-					if mask&^desired.avoid&^s.nospill != 0 {
+					if mask&^desired.avoid&^s.nospill&^s.used != 0 {
 						mask &^= desired.avoid
 					}
 					r := s.allocReg(mask, v)