There were two bugs here that were masking each other:

1. Case nodes were not being added to AST traversal path.
2. RecordConstantPrecision did not check if a constant was a child of
   a case node.

Fix these.

BUG=angleproject:1490
TEST=WebGL 2 conformance tests

Change-Id: Ic6cb7942bf57870fa26eb6a011919a2d7951cc85
Reviewed-on: https://chromium-review.googlesource.com/394588Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>