cmd/compile: don't panic when printing package-less symbols

cmd/compile: don't panic when printing package-less symbols

Some of the SSA pseudo-variables like the memory variable don't
have a package. Print those gracefully instead of printing a panic.

Fixes #51108

Change-Id: I5c29029356e045c5cf70909d6e63666ebc58ffaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/384614
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>