i965: Lower the GLSL ES 3.00 pack/unpack operations (v2)

i965: Lower the GLSL ES 3.00 pack/unpack operations (v2)

On gen < 7, we fully lower all operations to arithmetic and bitwise
operations.

On gen >= 7, we fully lower the Snorm2x16 and Unorm2x16 operations, and
partially lower the Half2x16 operations.

v2:
  - Comment that scalarization is needed only for SOA code [for idr].
  - Replace switch-statement with if-statement [for idr].
  - Remove misplaced hunk from previous patch [found by idr].

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Tuner <mattst88@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>