[dev.regabi] cmd/compile: separate out address taken computation from typechecker

This CL computes a second parallel addrtaken bit that we check
against the old way of doing it. A subsequent CL will rip out the
typechecker code and just use the new way.

Change-Id: I62b7342c44f694144844695386f80088bbd40bf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/275695


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>