Vect: Try the pattern of vector signed integer SAT_SUB

Almost the same as vector unsigned integer SAT_SUB, try to match
the signed version during the vector pattern matching.

The below test suites are passed for this patch.
* The rv64gcv fully regression test.
* The x86 bootstrap test.
* The x86 fully regression test.

gcc/ChangeLog:

	* tree-vect-patterns.cc (gimple_signed_integer_sat_sub): Add new
	func decl for signed SAT_SUB.
	(vect_recog_sat_sub_pattern_transform): Update comments.
	(vect_recog_sat_sub_pattern): Try the vector signed SAT_SUB
	pattern.

Signed-off-by: Pan Li <pan2.li@intel.com>

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