nir/algebraic: Separate ffma lowering from fusing

The i965 driver has its own pass for fusing mul+add combinations that's
much smarter than what nir_opt_algebraic can do so we don't want to get the
nir_opt_algebraic one just because we didn't set lower_ffma.

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

src/compiler/nir/nir.h

@@ -1624,6 +1624,7 @@ typedef struct nir_function {
 typedef struct nir_shader_compiler_options {
    bool lower_fdiv;
    bool lower_ffma;
+   bool fuse_ffma;
    bool lower_flrp32;
    /** Lowers flrp when it does not support doubles */
    bool lower_flrp64;