nir: Add an ALU lowering pass for mul_high.

This is based on the glsl/lower_instructions.cpp implementation, but
should be much more readable.

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

src/compiler/nir/meson.build

@@ -104,6 +104,7 @@ files_libnir = files(
   'nir_liveness.c',
   'nir_loop_analyze.c',
   'nir_loop_analyze.h',
+  'nir_lower_alu.c',
   'nir_lower_alu_to_scalar.c',
   'nir_lower_alpha_test.c',
   'nir_lower_atomics_to_ssbo.c',