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>

src/compiler/glsl/ir_optimization.h

@@ -48,6 +48,7 @@
 #define REVERSE_TO_SHIFTS         0x10000
 #define FIND_LSB_TO_FLOAT_CAST    0x20000
 #define FIND_MSB_TO_FLOAT_CAST    0x40000
+#define IMUL_HIGH_TO_MUL          0x80000
 
 /**
  * \see class lower_packing_builtins_visitor