cmd/compile/internal/gc: remove OHMUL Op

cmd/compile/internal/gc: remove OHMUL Op

Previously the compiler rewrote constant division into OHMUL
operations, but that rewriting was moved to SSA in CL 37015. Now OHMUL
is unused, so we can get rid of it.

Change-Id: Ib6fc7c2b6435510bafb5735b3b4f42cfd8ed8cdb
Reviewed-on: https://go-review.googlesource.com/37750
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>