cmd/compile: don't include instantiating types in type hash

This CL is a bit overkill, but it is pretty safe for 1.18. We'll
want to revisit for 1.19 so we can avoid the hash collisions between
types, e.g. G[int] and G[float64], that will cause some slowdowns
(but not incorrect behavior). Thanks Cherry for the simple idea.

Fixes #51250

Change-Id: I68130e09ba68e7cc35687bc623f63547bc552867
Reviewed-on: https://go-review.googlesource.com/c/go/+/389474


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>