Unsigned integers in GLSL were being parsed using the regular
integer parser, so it was limited to INT_MAX. All values from
INT_MAX + 1 to UINT_MAX could not be parsed properly.

Also, added constant folding for the 4 bit conversion glsl
functions.

Fixes shader compilation issue in the Epic Zen Garden example:
https://s3.amazonaws.com/mozilla-games/ZenGarden/EpicZenGarden.html
(unfortunately, the screen is still black, so there are other
 issues left)

Fixes WebGL 2 test: conformance2/glsl3/float-parsing.html

Change-Id: Iae52b2c8e083f0e1a22599e5a583297b9850444d
Reviewed-on: https://swiftshader-review.googlesource.com/16648Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>