cmd/compile: add rule location to some rulegen logging
cmd/compile: add rule location to some rulegen logging

This requires threading location information through varCount.

This provides much more useful error messages.

Change-Id: If5ff942cbbbf386724eda15a523c181c137fac20
Reviewed-on: https://go-review.googlesource.com/c/go/+/216221
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

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