tree-optimization/115395 - wrong-code with SLP reduction in epilog

tree-optimization/115395 - wrong-code with SLP reduction in epilog

When we continue a non-SLP reduction from the main loop in the
epilog with a SLP reduction we currently fail to handle an
adjustment by the initial value because that's not a thing with SLP.
As long as we have the possibility to mix SLP and non-SLP we have
to handle it though.

	PR tree-optimization/115395
	* tree-vect-loop.cc (vect_create_epilog_for_reduction):
	Handle STMT_VINFO_REDUC_EPILOGUE_ADJUSTMENT also for SLP
	reductions of group_size one.

	* gcc.dg/vect/pr115395.c: New testcase.