Fix folding of VECTOR_CST comparisons

Fix folding of VECTOR_CST comparisons

gcc/ChangeLog:

	PR tree-optimization/71077
	PR tree-optimization/68542
	* fold-const.c (fold_relational_const): Fix folding of
	VECTOR_CST comparisons that have a scalar boolean result type.
	(selftest::test_vector_folding): New static function.
	(selftest::fold_const_c_tests): Call it.

gcc/testsuite/ChangeLog:

	PR tree-optimization/71077
	* gcc.target/i386/pr71077.c: New test.

From-SVN: r239798