nir/flrp: Add new lowering pass for flrp instructions

This pass will soon grow to include some optimizations that are
difficult or impossible to implement correctly within nir_opt_algebraic.
It also include the ability to generate strictly correct code which the
current nir_opt_algebraic lowering lacks (though that could be changed).

v2: Document the parameters to nir_lower_flrp.  Rebase on top of
3766334923 ("compiler/nir: add lowering for 16-bit flrp")

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/nir/nir.h

@@ -3174,6 +3174,10 @@ bool nir_lower_vec_to_movs(nir_shader *shader);
 void nir_lower_alpha_test(nir_shader *shader, enum compare_func func,
                           bool alpha_to_one);
 bool nir_lower_alu(nir_shader *shader);
+
+bool nir_lower_flrp(nir_shader *shader, unsigned lowering_mask,
+                    bool always_precise, bool have_ffma);
+
 bool nir_lower_alu_to_scalar(nir_shader *shader);
 bool nir_lower_bool_to_float(nir_shader *shader);
 bool nir_lower_bool_to_int32(nir_shader *shader);