cmd/internal/gc: add backend ginscmp function to emit a comparison

cmd/internal/gc: add backend ginscmp function to emit a comparison

This lets us abstract away which arguments can be constants and so on
and lets the back ends reverse the order of arguments if that helps.

Change-Id: I283ec1d694f2dd84eba22e5eb4aad78a2d2d9eb0
Reviewed-on: https://go-review.googlesource.com/9810
Reviewed-by: David Chase <drchase@google.com>