cmd/compiler: make decompose shortcuts apply for PtrShaped, not just Ptr

cmd/compiler: make decompose shortcuts apply for PtrShaped, not just Ptr

The immediate-data interface shortcuts also apply to pointer-shaped
things like maps, channels, and functions.

Fixes #63505.

Change-Id: I43982441bf523f53e9e5d183e95aea7c6655dd6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/534657
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>