intel/fs: Fix IsHelperInvocation for the case no discard/demote are used

Use emit_predicate_on_sample_mask() helper that does check where to
get the correct mask depending on whether discard/demote was used or
not.

Fixes: 45f5db5a84 ("intel/fs: Implement "demote to helper invocation"")
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15400>

src/intel/compiler/brw_fs.h

@@ -203,6 +203,7 @@ public:
    void emit_dummy_fs();
    void emit_repclear_shader();
    void emit_fragcoord_interpolation(fs_reg wpos);
+   void emit_is_helper_invocation(fs_reg result);
    fs_reg emit_frontfacing_interpolation();
    fs_reg emit_samplepos_setup();
    fs_reg emit_sampleid_setup();