tree-optimization/110381 - preserve SLP permutation with in-order reductions

The following fixes a bug that manifests itself during fold-left
reduction transform in picking not the last scalar def to replace
and thus double-counting some elements.  But the underlying issue
is that we merge a load permutation into the in-order reduction
which is of course wrong.

Now, reduction analysis has not yet been performend when optimizing
permutations so we have to resort to check that ourselves.

	PR tree-optimization/110381
	* tree-vect-slp.cc (vect_optimize_slp_pass::start_choosing_layouts):
	Materialize permutes before fold-left reductions.

	* gcc.dg/vect/pr110381.c: New testcase.