Fixup partial_vectors_supported_p use

Fixup partial_vectors_supported_p use

The following fixes the computation of supports_partial_vectors which
is used to prune the set of modes to iterate over for epilog
vectorization.  The used partial_vectors_supported_p predicate
only looks for while_ult while also support predication when
mask modes are integer modes as for AVX512.

I've noticed this isn't very effective on x86_64 anyway since
if the main loop mode is autodetected we skip re-analyzing
mode_i == 0, but then mode_i == 1 is usually the very same
large mode.  A patch for this will follow, but this will
regress without the fix below.

	* tree-vect-loop.cc (vect_analyze_loop): Consider AVX512
	style masking when computing supports_partial_vectors.