[dev.typeparams] cmd/compile: cache mapped types during irgen

If we see the exact same types2.Type a second time, we can map it to
the same *types.Type instance. Not strictly necessary, but reduces
memory usage and plays better with the rest of the compiler given the
current state of things.

Change-Id: I53686d072c7c7834b0c97417bc8d5f2cd24572b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/284692


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>