Fixes #10994

CASE and BCASE were used by 7c in switch statements, cmd/compile
does not use them, cmd/assemble couldn't assemble them, and the arm64
peephole optimiser didn't know about them.

Change-Id: Id04835fcb37e207f76d211ce54a4db9c057d6112
Reviewed-on: https://go-review.googlesource.com/14100Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Run-TryBot: Aram Hăvărneanu <aram@mgk.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>