nir: Add alpha_to_coverage lowering pass

nir: Add alpha_to_coverage lowering pass

Importing this pass from fs_visitor::emit_alpha_to_coverage_workaround()
in intel/compiler.

v2 (Caio Marcelo de Oliveira Filho):
- Track store output and sample mask instruction
- Nest math insturction for more readability
- Bail out early if no gl_SampleMask

v3: (Caio Marcelo de Oliveira Filho):
- Do math instructions after instruction block
- Restructure code
- Move pass under src/intel/compiler

v4: (Caio Marcelo de Oliveira Filho):
- Organize dither mask calculation

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>