cmd/compile: remove dead values after flagalloc

cmd/compile: remove dead values after flagalloc

Fix a longstanding TODO.

Provides widespread, minor improvements.
Negligible compiler cost.

Because the freeze nears, put in a safety flag to easily disable.

Change-Id: I338812181ab6d806fecf22afd3c3502e2c94f7a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/229600
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>