This will allow us to disable optimizations in the back-end. This can
be useful both for developers and for ANGLE to disable very slow
shader compilation on D3D11.

Also apply this pragma to VerifyMaxVertexUniformVectorsWithSamplers.
Reduces compilation time by half in local testing.

Bug: angleproject:5076
Change-Id: I64ad576e11b9cee5b41f8af0d3621570304d65c2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2420749
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>