ifcvt/vect: Emit COND_OP for conditional scalar reduction.

ifcvt/vect: Emit COND_OP for conditional scalar reduction.

As described in PR111401 we currently emit a COND and a PLUS expression
for conditional reductions.  This makes it difficult to combine both
into a masked reduction statement later.
This patch improves that by directly emitting a COND_ADD/COND_OP during
ifcvt and adjusting some vectorizer code to handle it.

It also makes neutral_op_for_reduction return -0 if HONOR_SIGNED_ZEROS
is true.

gcc/ChangeLog:

	PR middle-end/111401
	* internal-fn.cc (internal_fn_else_index): New function.
	* internal-fn.h (internal_fn_else_index): Define.
	* tree-if-conv.cc (convert_scalar_cond_reduction): Emit COND_OP
	if supported.
	(predicate_scalar_phi): Add whitespace.
	* tree-vect-loop.cc (fold_left_reduction_fn): Add IFN_COND_OP.
	(neutral_op_for_reduction): Return -0 for PLUS.
	(check_reduction_path): Don't count else operand in COND_OP.
	(vect_is_simple_reduction): Ditto.
	(vect_create_epilog_for_reduction): Fix whitespace.
	(vectorize_fold_left_reduction): Add COND_OP handling.
	(vectorizable_reduction): Don't count else operand in COND_OP.
	(vect_transform_reduction): Add COND_OP handling.
	* tree-vectorizer.h (neutral_op_for_reduction): Add default
	parameter.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-cond-reduc-in-order-2-signed-zero.c: New test.
	* gcc.target/riscv/rvv/autovec/cond/pr111401.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_call-2.c: Adjust.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_call-4.c: Ditto.