cmd/compile: regalloc: handle desired registers of 2-output insns

cmd/compile: regalloc: handle desired registers of 2-output insns

Particularly with 2-word load instructions, this becomes important.
Classic example is:

    func f(p *string) string {
        return *p
    }

We want the two loads to put the return values directly into
the two ABI return registers.

At this point in the stack, cmd/go is 1.1% smaller.

Change-Id: I51fd1710238e81d15aab2bfb816d73c8e7c207b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/631137
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>