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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

CAFxX/go,akutz/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,golang/go,CAFxX/go,akutz/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,akutz/go,akutz/go,cloudflare/go,nawawi/go