tree-optimization/112827 - corrupt SCEV cache during SCCP

tree-optimization/112827 - corrupt SCEV cache during SCCP

The following avoids corrupting the SCEV cache by my last change
to propagate constant final values immediately.  The easiest fix
is to keep a dead initialization around.

	PR tree-optimization/112827
	* tree-scalar-evolution.cc (final_value_replacement_loop):
	Do not release SSA name but keep a dead initialization around.

	* gcc.dg/torture/pr112827-1.c: New testcase.
	* gcc.dg/torture/pr112827-2.c: Likewise.