The translator already prunes no-ops and unreferenced variables, and
this may result in case statements that are followed by nothing.

Since the last case statement in as switch statement must always
contain a statement to be valid GLSL, the translator must not leave
empty case statements in place. They're now being pruned by the
PruneEmptyCases AST transformation. This improves on the earlier
RemoveEmptySwitchStatements AST transformation that did address
empty switch statements but could not remove them if they had a
case statement inside.

BUG=angleproject:2402
TEST=angle_unittests

Change-Id: Ieb9598a744078e45226d8fb7266d877f7835cf0c
Reviewed-on: https://chromium-review.googlesource.com/962181
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>