vect: Fix epilogue loop handling of partial vectors

This patch fixes the fallout that Kewen reported on Power after
the recent change to avoid unnecessary use of partial vectors.
As Kewen said, the problem is that vect_analyze_loop_2 doesn't
know how many epilogue iterations there will be, and so it
cannot make a final decision about whether the number of
iterations forces an epilogue loop to use partial vectors.

This is similar to the current situation for peeling: we don't know
during initial analysis whether an epilogue loop will itself require
peeling.  Instead we decide that during vect_do_peeling, where the
final number of epilogue loop iterations is known.

The patch takes a similar approach for the decision about whether
to use partial vectors.  As the comments in the patch say, the
idea is that vect_analyze_loop_2 should make peeling and partial-
vector decisions based on the assumption that the loop_vinfo will
be used as the main loop, while vect_do_peeling should make them
in the knowledge that the loop_vinfo will be used as an epilogue loop.

This allows the same analysis to be used for both cases, which we
rely on for implementing VECT_COMPARE_COSTS; see the big comment
in vect_analyze_loop for details.

I hope the patch makes the (mostly preexisting) structure a bit
more obvious.  It isn't what anyone would design from scratch,
but that's the nature of working with a mature vector framework.

Arranging things this way means that vect_verify_full_masking
and vect_verify_loop_lens now become part of the “can” rather
than “will” test for partial vectors.

Also, while splitting out the logic that handles epilogues with
constant iterations, I added a check to make sure that we don't
try to use partial vectors to vectorise a single-scalar loop.
This required some changes to the Power tests.

gcc/
	* tree-vectorizer.h (determine_peel_for_niter): Delete in favor of...
	(vect_determine_partial_vectors_and_peeling): ...this new function.
	* tree-vect-loop-manip.c (vect_update_epilogue_niters): New function.
	Reject using vector epilogue loops for single iterations.  Install
	the constant number of epilogue loop iterations in the associated
	loop_vinfo.  Rely on vect_determine_partial_vectors_and_peeling
	to do the main part of the test.
	(vect_do_peeling): Use vect_update_epilogue_niters to handle
	epilogue loops with a known number of iterations.  Skip recomputing
	the number of iterations later in that case.  Otherwise, use
	vect_determine_partial_vectors_and_peeling to decide whether the
	epilogue loop needs to use partial vectors or peeling.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Set the
	default can_use_partial_vectors_p to false if partial-vector-usage=0.
	(determine_peel_for_niter): Remove in favor of...
	(vect_determine_partial_vectors_and_peeling): ...this new function,
	split out from...
	(vect_analyze_loop_2): ...here.  Reflect the vect_verify_full_masking
	and vect_verify_loop_lens results in CAN_USE_PARTIAL_VECTORS_P
	rather than USING_PARTIAL_VECTORS_P.

gcc/testsuite/
	* gcc.target/powerpc/p9-vec-length-epil-1.c: Do not expect the
	single-iteration epilogues of the 64-bit loops to be vectorized.
	* gcc.target/powerpc/p9-vec-length-epil-7.c: Likewise.
	* gcc.target/powerpc/p9-vec-length-epil-8.c: Likewise.

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