tree-optimization/122474 - adjust check for .VEC_SHL_INSERT

tree-optimization/122474 - adjust check for .VEC_SHL_INSERT

The r16-4558-g1b387bd8978577 change added a check that does not
match what the commit message says.  The following fixes this,
trying to more closely match up what we later do during transform
and what we check here.  As cleanup this also makes sure that
we compute the neutral value for the scalar type and consistently
when it depends on the initial value, recording it in a new
VECT_REDUC_INFO_NEUTRAL_OP.  This avoids issues with comparing
the neutral and initial value when it is bool.  This also
refactors vect_transform_cycle_phi a bit to remove dead code
and make the flow easier to follow.

	PR tree-optimization/122474
	* tree-vectorizer.h (vect_reduc_info_s::neutral_op): New.
	(VECT_REDUC_INFO_NEUTRAL_OP): New.
	* tree-vect-loop.cc (vectorizable_reduction): Adjust condition
	guarding the check for .VEC_SHL_INSERT.

	* gcc.target/aarch64/sve2/pr123053.c: New testcase.
	* gcc.target/riscv/rvv/pr122474.c: Likewise.