vect: Rename can_fully_mask_p to can_use_partial_vectors_p

vect: Rename can_fully_mask_p to can_use_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 pointed out, we should extend the existing flag
can_fully_mask_p to be more generic, to indicate whether we have
any chances with partial vectors for this loop.  So this patch
is to rename this flag to can_use_partial_vectors_p to be more
meaningful, also rename the macro LOOP_VINFO_CAN_FULLY_MASK_P
to LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P.

Bootstrapped/regtested on aarch64-linux-gnu.

gcc/ChangeLog:

	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Rename
	can_fully_mask_p to can_use_partial_vectors_p.
	(vect_analyze_loop_2): Rename LOOP_VINFO_CAN_FULLY_MASK_P to
	LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P.  Rename saved_can_fully_mask_p
	to saved_can_use_partial_vectors_p.
	(vectorizable_reduction): Rename LOOP_VINFO_CAN_FULLY_MASK_P to
	LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P.
	(vectorizable_live_operation): Likewise.
	* tree-vect-stmts.c (permute_vec_elements): Likewise.
	(check_load_store_masking): Likewise.
	(vectorizable_operation): Likewise.
	(vectorizable_store): Likewise.
	(vectorizable_load): Likewise.
	(vectorizable_condition): Likewise.
	* tree-vectorizer.h (LOOP_VINFO_CAN_FULLY_MASK_P): Renamed to ...
	(LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P): ... this.
	(_loop_vec_info): Rename can_fully_mask_p to can_use_partial_vectors_p.