glsl: Add lowering pass for GLSL ES 3.00 pack/unpack operations (v4)

glsl: Add lowering pass for GLSL ES 3.00 pack/unpack operations (v4)

Lower them to arithmetic and bit manipulation expressions.

v2: Rewrite using ir_builder [for idr].
v3: Comment typos. [for mattst88]
v4: Fix arithmetic error in comments.
    Factor out a shift instruction.
    Don't heap allocate factory.instructions.
    [for paul]

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v2)
Reviewed-by: Matt Tuner <mattst88@gmail.com> (v3)
Reviewed-by: Paul Berry <stereotype441@gmail.com> (v4)
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>