If variables are declared inside a GLSL switch statement, they are
scoped until the end of the switch statement. This is not compatible
with HLSL rules, where the scoping is until the end of the case. To
work around this, wrap switch statements in a block that declares
the variables in HLSL.

This is done after most other transformations done to the AST are
complete, since some of the other transformations may introduce
temporary variables.

BUG=angleproject:2179
TEST=angle_end2end_tests

Change-Id: Id0bb89affe103177fd3d6a6b2f3619b5e1ada0a6
Reviewed-on: https://chromium-review.googlesource.com/716381Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>