cmd/compile: make the stack allocator more careful about register args.

Assignment between input parameters causes them to have more than
one "Name", and running this backwards from names to values can end
up confusing (conflating) parameter spill slots.

Around 105a6e95, this cases a stack overflow running
go test -race encoding/pem
because two slice parameters spill (incorrectly) into the same
stack slots (in the AB?I-defined parameter spill area).

This also tickles a failure in cue, which turned out to be
easier to isolate.

Fixes #45851.
Updates #40724.

Change-Id: I39c56815bd6abb652f1ccbe83c47f4f373a125c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/313212


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>