i965/vs/gen7: Emit code for GLSL ES 3.00 pack/unpack operations (v3)

i965/vs/gen7: Emit code for GLSL ES 3.00 pack/unpack operations (v3)

FIXME: This patch emits VS code that violates documented hardware
restrictions and then relies on undocumented behavior that results from
that violation.  This patch passes all tests, but should be fixed ASAP to
conform to the hardware documentation.

v2: Explain undocumented hardware behavior. Improve comments.
v3: Use ALU1 helper methods F32TO16() and F16TO32(). [for anholt]

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>