amdgcn: vector testsuite tweaks

amdgcn: vector testsuite tweaks

The testsuite needs a few tweaks following my patches to add multiple vector
sizes for amdgcn.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr104464.c: Xfail on amdgcn.
	* gcc.dg/signbit-2.c: Likewise.
	* gcc.dg/signbit-5.c: Likewise.
	* gcc.dg/vect/bb-slp-68.c: Likewise.
	* gcc.dg/vect/bb-slp-cond-1.c: Change expectations on amdgcn.
	* gcc.dg/vect/bb-slp-subgroups-3.c: Likewise.
	* gcc.dg/vect/no-vfa-vect-depend-2.c: Change expectations for multiple
	vector sizes.
	* gcc.dg/vect/pr33953.c: Likewise.
	* gcc.dg/vect/pr65947-12.c: Likewise.
	* gcc.dg/vect/pr65947-13.c: Likewise.
	* gcc.dg/vect/pr80631-2.c: Likewise.
	* gcc.dg/vect/slp-reduc-4.c: Likewise.
	* gcc.dg/vect/trapv-vect-reduc-4.c: Likewise.
	* lib/target-supports.exp (available_vector_sizes): Add more sizes
	for amdgcn.