cmd/compile: amd64, allow XCHG on stack pointers

cmd/compile: amd64, allow XCHG on stack pointers

XCHG needs to allow the stack pointer as an argument because we have a
rewrite that incorporates the address of a local variable into the
instruction.

Fixes #19184

Change-Id: Ic438e6e1946332cdce3864d15abecd41b911b2a9
Reviewed-on: https://go-review.googlesource.com/37253
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>