Enhance vec_pack_trunc for integral mode mask.

Enhance vec_pack_trunc for integral mode mask.

For testcase in PR, the patch supports QI:4 -> HI:16 pack with
multi steps(first pack QI:4 -> QI:8 through vec_pack_sbool_trunc_qi,
then pack QI:8 -> HI:16 through vec_pack_trunc_hi).
Similar for QI:2 -> HI:16 which is test4 in mask-pack-prefer-128.c.

gcc/ChangeLog:

	PR target/103771
	* tree-vect-stmts.cc (supportable_narrowing_operation): Enhance
	integral mode mask pack by multi steps which takes
	vec_pack_sbool_trunc_optab as start when elements number is
	less than BITS_PER_UNITS.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/mask-pack-prefer128.c: New test.
	* gcc.target/i386/mask-pack-prefer256.c: New test.
	* gcc.target/i386/pr103771.c: New test.