cmd/compile/internal/ssa: use Compare instead of Equal

cmd/compile/internal/ssa: use Compare instead of Equal

They have different semantics.

Equal is stricter and is designed for the front-end.
Compare is looser and cheaper and is designed for the back-end.
To avoid possible regression, remove Equal from ssa.Type.

Updates #15043

Change-Id: Ie23ce75ff6b4d01b7982e0a89e6f81b5d099d8d6
Reviewed-on: https://go-review.googlesource.com/21483
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>