Disallow scan-store vectorization in epilogues

Disallow scan-store vectorization in epilogues

The following disallows vectorizing epilogues containing scan-stores.
Since code generation works by walking gimple stmts it is not ready
for this when cleaning up epilogue vectorization.  I believe
scan-store vectorization needs most of the work done during SLP
discovery to reflect the data flow.

	* tree-vect-stmts.cc (check_scan_store): Remove redundant
	slp_node check.  Disallow epilogue vectorization.