glsl: Add lowering pass for ir_triop_vector_insert

glsl: Add lowering pass for ir_triop_vector_insert

This will eventually replace do_vec_index_to_cond_assign.  This lowering
pass is called in all the places where do_vec_index_to_cond_assign or
do_vec_index_to_swizzle is called.

v2: Use WRITEMASK_* instead of integer literals.  Use a more concise
method of generating broadcast_index.  Both suggested by Eric.

v3: Use a series of scalar compares instead of a single vector compare.
Suggested by Eric and Ken.  It still uses 'if (cond) v.x = y;' instead
of conditional assignments because ir_builder doesn't do conditional
assignments, and I'd rather keep the code simple.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>