middle-end/123107 - avoid invalid vector folding

middle-end/123107 - avoid invalid vector folding

We fold (v >> CST) == { 0, 0.. } into v < { 0, 0.. } but fail to
validate that's valid for the target.  The following adds such check,
making sure to apply after IPA (due to offloading) and only when
the original form wasn't valid for the target (like before vector
lowering) or when the new form is.  In particular in this case
we have an equality compare resulting in a non-vector which we
can handle, but a similar LT/GT is never handled.

	PR middle-end/123107
	* fold-const.cc (fold_binary_loc): Guard (v >> CST) == { 0, 0.. }
	to v < { 0, 0.. } folding.

	* gcc.dg/torture/pr123107.c: New testcase.