Clean up RecordConstantPrecision interaction with switch/case 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/394588 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>