[dev.regabi] cmd/compile: separate ssa from other phases

isIntrinsicCall and ssaDumpInline are the only two "forward references"
to ssa by earlier phases. Make them a bit more explicit so that the
uses and the definitions can end up in different packages.

Change-Id: I02c7a27464fbedef9fee43c0e4094fa08b4d7a5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/279300


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>