There was some duplicate switch/case pruning in the code in
PruneEmptyCases and RemoveNoOpStatementsFromTheEndOfSwitchStatements.
Combine the functionality of both AST transformations into
PruneEmptyCases and remove the other transformation.

The tests are improved to better cover the full functionality.

BUG=angleproject:2402
TEST=angle_unittests, angle_end2end_tests --gtest_filter=*Switch*

Change-Id: Ib74b6b9b455769ea15650e9653a9c53635342c49
Reviewed-on: https://chromium-review.googlesource.com/964081Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>