testsuite: Add a vect_partial_vectors_usage_2 guard

testsuite: Add a vect_partial_vectors_usage_2 guard

We don't need an epilogue loop if the main loop can operate on
partial vectors, so this patch disables an associated test.
The alternative would be to force partial-vectors-usage=1
on the command line.

gcc/testsuite/
	* gcc.dg/vect/vect-epilogues.c: XFAIL test for epilogue loop
	vectorization if vect_partial_vectors_usage_2.