intel/compiler: extract subfunctions of lower_integer_multiplication()

The lower_integer_multiplication() function is already a little too
big. I want to add more to it, so let's reorganize the existing code
first. Let's start with just extracting the current code to
subfunctions. Later we'll change them a little more.

v2: Make private functions private (Caio).
v3: Fix typo (Caio).

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>

src/intel/compiler/brw_fs.h

@@ -406,6 +406,10 @@ private:
 
    void resolve_inot_sources(const brw::fs_builder &bld, nir_alu_instr *instr,
                              fs_reg *op);
+   void lower_mul_dword_inst(fs_inst *inst, bblock_t *block,
+                             const brw::fs_builder &ibld);
+   void lower_mulh_inst(fs_inst *inst, bblock_t *block,
+                        const brw::fs_builder &ibld);
 };
 
 /**