intel/compiler: Remove emit_alpha_to_coverage workaround from backend

Remove emit_alpha_to_coverage workaround from backend compiler and start
using ported workaround from NIR.

v2: Copy comment from brw_fs_visitor (Caio Marcelo de Oliveira Filho)

Fixes piglit test on HSW:
- arb_sample_shading-builtin-gl-sample-mask-mrt-alpha-to-coverage-combinations

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

src/intel/compiler/brw_fs.cpp

@@ -8241,6 +8241,19 @@ brw_compile_fs(const struct brw_compiler *compiler, void *log_data,
    if (devinfo->gen < 6)
       brw_setup_vue_interpolation(vue_map, shader, prog_data);
 
+   /* From the SKL PRM, Volume 7, "Alpha Coverage":
+    *  "If Pixel Shader outputs oMask, AlphaToCoverage is disabled in
+    *   hardware, regardless of the state setting for this feature."
+    */
+   if (devinfo->gen > 6 && key->alpha_to_coverage) {
+      /* Run constant fold optimization in order to get the correct source
+       * offset to determine render target 0 store instruction in
+       * emit_alpha_to_coverage pass.
+       */
+      NIR_PASS_V(shader, nir_opt_constant_folding);
+      NIR_PASS_V(shader, brw_nir_lower_alpha_to_coverage);
+   }
+
    if (!key->multisample_fbo)
       NIR_PASS_V(shader, demote_sample_qualifiers);
    NIR_PASS_V(shader, move_interpolation_to_top);