vect: Merge loop mask and cond_op mask in fold-left reduction [PR115382].

vect: Merge loop mask and cond_op mask in fold-left reduction [PR115382].

Currently we discard the cond-op mask when the loop is fully masked
which causes wrong code in
gcc.dg/vect/vect-cond-reduc-in-order-2-signed-zero.c
when compiled with
-O3 -march=cascadelake --param vect-partial-vector-usage=2.

This patch ANDs both masks.

gcc/ChangeLog:

	PR tree-optimization/115382

	* tree-vect-loop.cc (vectorize_fold_left_reduction): Use
	prepare_vec_mask.
	* tree-vect-stmts.cc (check_load_store_for_partial_vectors):
	Remove static of prepare_vec_mask.
	* tree-vectorizer.h (prepare_vec_mask): Export.