vect: Rename fully_masked_p to using_partial_vectors_p

vect: Rename fully_masked_p to using_partial_vectors_p

Power supports vector memory access with length (in bytes) instructions.
Like existing fully masking for SVE, it is another approach to vectorize
the loop using partially-populated vectors.

As Richard Sandiford suggested, this patch is to update the existing
fully_masked_p field to using_partial_vectors_p.  Introduce one macro
LOOP_VINFO_USING_PARTIAL_VECTORS_P for partial vectorization checking
usage, update the LOOP_VINFO_FULLY_MASKED_P with
LOOP_VINFO_USING_PARTIAL_VECTORS_P && !masks.is_empty() and still use
it for mask-based partial vectors approach specific checks.

Bootstrapped/regtested on aarch64-linux-gnu.

gcc/ChangeLog:

	* tree-vect-loop-manip.c (vect_set_loop_condition): Rename
	LOOP_VINFO_FULLY_MASKED_P to LOOP_VINFO_USING_PARTIAL_VECTORS_P.
	(vect_gen_vector_loop_niters): Likewise.
	(vect_do_peeling): Likewise.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Rename
	fully_masked_p to using_partial_vectors_p.
	(vect_analyze_loop_costing): Rename LOOP_VINFO_FULLY_MASKED_P to
	LOOP_VINFO_USING_PARTIAL_VECTORS_P.
	(determine_peel_for_niter): Likewise.
	(vect_estimate_min_profitable_iters): Likewise.
	(vect_transform_loop): Likewise.
	* tree-vectorizer.h (LOOP_VINFO_FULLY_MASKED_P): Updated.
	(LOOP_VINFO_USING_PARTIAL_VECTORS_P): New macro.