glsl: Add lowering pass for ir_bin_imul_high

glsl: Add lowering pass for ir_bin_imul_high

This isn't the lowering pass you want.  Most GPUs that can support GLSL
1.30 have a multiply unit that can do something more interesting than
32x32->32.  Many have 32x16->48.  Any GPU that does, should do the
lowering in the backend.  This is just the thing that will always work.

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