cmd/compile: fix pointer maps for morestack

Verified with test and with single step watching changes to register
values across morestack calls, after reload.

Also added stack-growth test with pointer parameters of varying lifetime.

For #40724.

Change-Id: Idb5fe27786ac5c6665a734d41e68d3d39de2f4da
Reviewed-on: https://go-review.googlesource.com/c/go/+/294429


Trust: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>