Instead of storing a type GLenum, store a pointer into the type info
table. This makes looking up some values a bit easier.
Also includes some micro-optimizations.

Improves the score of the uniforms micro-benchmark by about 12.4% on
a local Windows 10 machine.

BUG=angleproject:1390

Change-Id: I35f7f1c7bb3cf2c62d737b494dc56ff61ea2d1fb
Reviewed-on: https://chromium-review.googlesource.com/659221
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>