tree-optimization/111125 - properly cost BB reduction remain stmt handling

tree-optimization/111125 - properly cost BB reduction remain stmt handling

We assume that all root stmts which compose the total reduction chain
are vectorized but fail to account for the cost of adding back the
scalar defs we are not vectorizing.  The following rectifies this,
fixing the gcc.dg/tree-ssa/slsr-11.c FAIL on aarch64.

	PR tree-optimization/111125
	* tree-vect-slp.cc (vectorizable_bb_reduc_epilogue): Account
	for the remain_defs processing.