intel/fs: Add SHADER_OPCODE_[IU]SUB_SAT pseudo-ops

v2: Add a big comment explaining the [IU]SUB_SAT lowering.  Suggested by
Caio.

v3: Use get_fpu_lowered_simd_width in get_lowered_simd_width.  Suggested
by Ken on IRC.

v4: Fix a typo in a comment.  Noticed by Caio.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/767>

src/intel/compiler/brw_fs.h

@@ -169,6 +169,7 @@ public:
    bool lower_simd_width();
    bool lower_barycentrics();
    bool lower_scoreboard();
+   bool lower_sub_sat();
    bool opt_combine_constants();
 
    void emit_dummy_fs();