This way we can do numeric comparisons instead of string comparisons.

The effect on compiler perf test scores is fairly marginal, but
this reduces binary size by a few kilobytes, and there may be a larger
effect on shaders calling a lot of texture functions.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I077db97b16b16b70b7e18ee037e06d7450d08dc9
Reviewed-on: https://chromium-review.googlesource.com/947952
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>