Add a pls test for in-shader coherency Stores and loads in a single shader invocation should be coherent. Bug: angleproject:7279 Change-Id: I1a4af6fdbef21f5506bb40c9eb8f5e76c12dd6cd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3654267 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Chris Dalton <chris@rive.app>