[vectorizer][2/2] Hook up mult synthesis logic into vectorisation of mult-by-constant

[vectorizer][2/2] Hook up mult synthesis logic into vectorisation of mult-by-constant

	PR target/65951
	PR tree-optimization/70923
	* tree-vect-patterns.c: Include mult-synthesis.h.
	(target_supports_mult_synth_alg): New function.
	(synth_lshift_by_additions): Likewise.
	(apply_binop_and_append_stmt): Likewise.
	(vect_synth_mult_by_constant): Likewise.
	(target_has_vecop_for_code): Likewise.
	(vect_recog_mult_pattern): Use above functions to synthesize vector
	multiplication by integer constants.

	* gcc.dg/vect/vect-mult-const-pattern-1.c: New test.
	* gcc.dg/vect/vect-mult-const-pattern-2.c: Likewise.
	* gcc.dg/vect/pr65951.c: Likewise.
	* gcc.dg/vect/vect-iv-9.c: Remove ! vect_int_mult-specific scan.

From-SVN: r238340