The DynamicHLSL code was using global strings to find stubs.
The math code was also using pow in a global, which can be replaced
with a simple bit shift.

BUG=angleproject:1459

Change-Id: Idb0602ab7640c221843385b0a0a4cfecd5fd3a26
Reviewed-on: https://chromium-review.googlesource.com/403289Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>