glsl: Evaluate constant GLSL ES 3.00 pack/unpack expressions (v3)
glsl: Evaluate constant GLSL ES 3.00 pack/unpack expressions (v3)

That is, evaluate constant expressions of the following functions:
  packSnorm2x16  unpackSnorm2x16
  packUnorm2x16  unpackUnorm2x16
  packHalf2x16   unpackHalf2x16

v2: Reuse _mesa_pack_float_to_half and its inverse to evaluate
    pack/unpackHalf2x16. [for idr]
v3: Whitespace fixes. [for mattst88]
    Don't cast neg floats directly to uint16; use an intermediate cast to
    int16. [for paul]

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com> (v2)
Reviewed-by: Paul Berry <de21b9f22e6dd0bdf7890875d6e07ef4967c44ce@gmail.com>
Reviewed-by: Matt Tuner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>

mapbox/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,tokyovigilante/glsl-optimizer,jbarczak/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer