Built-ins with no math op associated with them now have the op code
EOpCallBuiltInFunction set. This makes initializing built-in function
nodes simpler, since they can always get the op code from the function
symbol.

We also no longer look for functions in inner scopes, only from the
global scope and from built-in functions.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I55a2642f34bb3c8b8f13183c95fa509ec3b9cfdb
Reviewed-on: https://chromium-review.googlesource.com/923724Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>