cmd/compile/internal/ssagen: add initial test coverage for intrinsics

Add basic test coverage for the intrinisic table - this at least allows
us to tell if intrinsics are added or removed unexpectedly. Code
generation changes resulting from intrinsics is not covered and is
left for test/codegen and others.

Change-Id: I3d538708b90cd04d3f449945e0fd9388097d683e
Reviewed-on: https://go-review.googlesource.com/c/go/+/605475


Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>