During AllocFrame, we drop unused variables from Curfn.Func.Dcl, but
there might still be OpVarFoo instructions that reference those
variables. This wasn't a problem though because gvardefx used to emit
ANOP for unused variables instead of AVARFOO.

As an easy fix, if we see OpVarFoo (or OpKeepAlive) referencing an
unused variable, we can ignore it.

Fixes #19632.

Change-Id: I4e9ffabdb4058f7cdcc4663b540f5a5a692daf8b
Reviewed-on: https://go-review.googlesource.com/38400Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>