[dev.regabi] cmd/compile: collect global compilation state

There are various global variables tracking the state of the
compilation. Collect them in a single global struct instead.
The struct definition is in package ir, but the struct itself is
still in package gc. It may eventually be threaded through the
code, but in the short term will end up in package typecheck.

Change-Id: I019db07aaedaed2c9b67dd45a4e138dc6028e54c
Reviewed-on: https://go-review.googlesource.com/c/go/+/279297


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>