[dev.regabi] cmd/compile: add register ABI analysis utilities

Introduce a new utility routine for analyzing a given function
signature to how its various input and output parameters will be
passed (in registers or on the stack) for a given ABI description,
along with some unit tests.

Change-Id: Id64a98a0a142e42dd9c2dc9f6607c0d827ef84fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/273011


Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Trust: Than McIntosh <thanm@google.com>