cmd/compile: remove OCASE and rename OXCASE to OCASE

We used to use OXCASE to represent general, possibly multi-valued
cases, and then desugar these during walk into single-value cases
represented by OCASE.

In CL 194660, we switched to eliminated the desugaring step and
instead handle the multi-valued cases directly, which eliminates the
need for an OCASE Op. Instead, we can simply remove OCASE, and rename
OXCASE to just OCASE.

Passes toolstash-check.

Change-Id: I3cc184340f9081d37453927cca1c059267fdbc12
Reviewed-on: https://go-review.googlesource.com/c/go/+/196117


Reviewed-by: Keith Randall <khr@golang.org>