The GLSL ES 3.00.6 spec is a bit unclear on this, but it does disallow
case statements "inside control flow". GLSL ES 3.10 clarifies this and
disallows any nesting of case or default labels within other
statements.

BUG=angleproject:2452
TEST=angle_unittests

Change-Id: I289bb39abb5227eab7117638af388b0a57dc5dd8
Reviewed-on: https://chromium-review.googlesource.com/995478
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>