Vect: Try the pattern of vector signed integer SAT_TRUNC

Almost the same as vector unsigned integer SAT_TRUNC, 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_trunc): Add
	new func decl for signed SAT_TRUNC.
	(vect_recog_sat_trunc_pattern): Try signed match pattern for
	the SAT_TRUNC.

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