This change adds constant folding support for following floating
point pack and unpack functions:
    - packSnorm2x16, unpackSnorm2x16, packUnorm2x16, unpackUnorm2x16,
      packHalf2x16, and unpackHalf2x16.

BUG=angleproject:913
TEST=angle_unittests(new: MathUtilTest.packAndUnpack*), dEQP Tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.float_pack_unpack.*
(all 10 tests started passing with this change)

Change-Id: I2b69faebee4127e5e13900b3a9485b7145950277
Reviewed-on: https://chromium-review.googlesource.com/282683Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>