cmd/internal/gc: show register dump before crashing on register left allocated

cmd/internal/gc: show register dump before crashing on register left allocated

If you are using -h to get a stack trace at the site of the failure,
Yyerror will never return. Dump the register allocation sites
before calling Yyerror.

Change-Id: I51266c03e06cb5084c2eaa89b367b9ed85ba286a
Reviewed-on: https://go-review.googlesource.com/9788
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Dave Cheney <dave@cheney.net>