cmd/compile: make pointers to arrays their own shape

Pointers to arrays can be used to cast from a slice. We need
the shape of such type params to be different so we can compile
those casts correctly.

This is kind of a big hammer to fix #49295. It would be nice to
only do this when we know there's a []T->*[N]T conversion.

Fixes #49295

Change-Id: Ibda33057fab2dd28162537aab0f1244211d68e3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/361135


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>