cmd/compile: revert CL/316890

This is a revert of https://go-review.googlesource.com/c/go/+/316890,
which has positive effects on debugging + DWARF variable locations
for register parameters when the reg abi is in effect, but also
turns out to interact badly with the register allocator.

Fixes #46304.

Change-Id: I624bd980493411a9cde45d44fcd3c46cad796909
Reviewed-on: https://go-review.googlesource.com/c/go/+/321830


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>