intel/fs/gen12: Implement gl_FrontFacing on gen12+.

The bit moved on gen12 in order to prepare for dual-SIMD8 dispatch.
This implementation isn't an entirely complete as it only works on SIMD8
and SIMD16 and not dual-SIMD8.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs.cpp

@@ -1326,7 +1326,13 @@ fs_visitor::emit_frontfacing_interpolation()
 {
    fs_reg *reg = new(this->mem_ctx) fs_reg(vgrf(glsl_type::bool_type));
 
-   if (devinfo->gen >= 6) {
+   if (devinfo->gen >= 12) {
+      fs_reg g1 = fs_reg(retype(brw_vec1_grf(1, 1), BRW_REGISTER_TYPE_W));
+
+      fs_reg tmp = bld.vgrf(BRW_REGISTER_TYPE_W);
+      bld.ASR(tmp, g1, brw_imm_d(15));
+      bld.NOT(*reg, tmp);
+   } else if (devinfo->gen >= 6) {
       /* Bit 15 of g0.0 is 0 if the polygon is front facing. We want to create
        * a boolean result from this (~0/true or 0/false).
        *