cmd/compile: prefer fixed registers for values

cmd/compile: prefer fixed registers for values

For this code:

func f() (int, int) {
	return 0, 0
}

We currently generate on arm64:

   MOVD ZR, R0
   MOVD R0, R1

This CL changes that to

   MOVD ZR, R0
   MOVD ZR, R1

Probably not a big performance difference, but it makes the generated
code clearer.

A followup-ish CL from 633075 when the zero fixed register was exposed
to the register allocator.

Change-Id: I869a92817dcbbca46c900999fab538e76e10ed05
Reviewed-on: https://go-review.googlesource.com/c/go/+/721440
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>