middle-end/123175 - fix parts of const VEC_PERM with relaxed input sizes

middle-end/123175 - fix parts of const VEC_PERM with relaxed input sizes

The following fixes enough of const VEC_PERM folding and lowering
to deal with the fallout for the two testcases from the PR.  We
usually do not generate such problematic VEC_PERM expressions, but
we allow those since GCC 14.  As can be seen we mishandle those,
including failure to expand/lower them by zero-extending inputs (which is
what __builtin_shufflevector does).

I'm unsure as to what extent we get such permutes but Tamar indicates
that aarch64 can handle those at least.

	PR middle-end/123175
	* match.pd (vec_perm @0 @1 @2): Fixup for inputs having a
	different number of elements than the result.
	* tree-vect-generic.cc (lower_vec_perm): Likewise.

	* gcc.dg/torture/pr123175-1.c: New testcase.
	* gcc.dg/torture/pr123175-2.c: Likewise.