i965/fs/gen7: Emit code for GLSL 3.00 pack/unpack operations (v4)

i965/fs/gen7: Emit code for GLSL 3.00 pack/unpack operations (v4)

v2: Remove lewd comment. [for idr]
v3: - Optimize away tmp register for packHalf2x16. [for anholt, paul]
    - Improve comments. [for anholt, paul]
    - Reduce near-duplicate code by removing vec4_visitor emit_pack/unpack
      methods. [for chadv]
v4: Factor our UD/W register conversion into helper function. [for anholt]

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v2)
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>