AST analysis to narrow down usage of [[loop]] and [[unroll]] failed to account
for break statements inside switch statements. Add switch statement handling.
This fixes asserts/crashes in dEQP tests.

BUG=angleproject:937
TEST=dEQP-GLES3.functional.shaders.switch.*

Change-Id: I04fdfe4733772a2a234934123bbfacf0376df562
Reviewed-on: https://chromium-review.googlesource.com/265191Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>