cmd/compile: always include underlying type for map types

This is a different fix for #37716.

Should help make the fix for #46283 easier, since we will no longer
need to keep compiler-generated hash functions and the runtime
hash function in sync.

Change-Id: I84cb93144e425dcd03afc552b5fbd0f2d2cc6d39
Reviewed-on: https://go-review.googlesource.com/c/go/+/322150


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>