glsl: Generate code for constant unary expressions that don't assign the destination

glsl: Generate code for constant unary expressions that don't assign the destination

These are operations like the pack functions that have separate
functions that assign multiple outputs from a single input.

v2: Correct the source and destination types.  They were previously
transposed.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>