Remove fall-through from non-empty labels in switch statements.

Tested with dEQP-GLES3.functional.shaders.*switch*. All pass except for tests
that require dFdx/dFdy, which fail for an unrelated reason.

Additional test coverage could still be useful for cases where a label is
terminated by a continue or return statement.

BUG=angle:921

Change-Id: I4741867789a9308d66d0adeabdaf83907106e2d2
Reviewed-on: https://chromium-review.googlesource.com/254550Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>