cmd/compile: make expandCalls preserve types of pointer stores

This is accomplished by checking for simple stores of pointer types
and leaving them alone.  The failure case was when a *mspan
(not in heap) stored type was replaced by unsafe.Pointer.

Updates #40724.

Change-Id: I529e1705bf58fb0e64e60d48fd550b3a407e57e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/305672


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>