glsl: Extend ir_expression_operation for GLSL 3.00 pack/unpack functions (v2)

glsl: Extend ir_expression_operation for GLSL 3.00 pack/unpack functions (v2)

For each function {pack,unpack}{Snorm,Unorm,Half}2x16, add a corresponding
opcode to enum ir_expression_operation.  Validate the new opcodes in
ir_validate.cpp.

Also, add opcodes for scalarized variants of the Half2x16 functions.  (The
code generator for the i965 fragment shader requires that all vector
operations be scalarized.  A lowering pass, to be added later, will
scalarize the Half2x16 functions).

v2: Fix assertion message in ir_to_mesa [for 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>