tree-optimization/121659 - bogus swap of reduction operands

The following addresses a bogus swapping of SLP operands of a
reduction operation which gets STMT_VINFO_REDUC_IDX out of sync
with the SLP operand order.  In fact the most obvious mistake is
that we simply swap operands even on the first stmt even when
there's no difference in the comparison operators (for == and !=
at least).  But there are more latent issues that I noticed and
fixed up in the process.

	PR tree-optimization/121659
	* tree-vect-slp.cc (vect_build_slp_tree_1): Do not allow
	matching up comparison operators by swapping if that would
	disturb STMT_VINFO_REDUC_IDX.  Make sure to only
	actually mark operands for swapping when there was a
	mismatch and we're not processing the first stmt.

	* gcc.dg/vect/pr121659.c: New testcase.

(cherry picked from commit 68e692eed9e8e8c47d83586ee08f40c27fa3a78d)