intel/fs: Use helper for discard sample mask flag subregister number.

Use it instead of hard-coding f0.1 for the sample mask of programs
that use discard.  This will make the task easier when we replace f0.1
with another flag register location in order to support discard with
SIMD32 shaders.

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

src/intel/compiler/brw_fs.h

@@ -417,6 +417,17 @@ private:
    unsigned workgroup_size() const;
 };
 
+/**
+ * Return the flag register used in fragment shaders to keep track of live
+ * samples.
+ */
+static inline unsigned
+sample_mask_flag_subreg(const fs_visitor *shader)
+{
+   assert(shader->stage == MESA_SHADER_FRAGMENT);
+   return 1;
+}
+
 /**
  * The fragment shader code generator.
  *