tree-optimization/118521 - bogus diagnostic from unreachable code

tree-optimization/118521 - bogus diagnostic from unreachable code

When SCCP does final value replacement we end up with unfolded IL like

__result_274 = _150 + 1;
...
__new_finish_106 = __result_274 + 3;  <-- from SCCP
_115 = _150 + 4;
if (__new_finish_106 != _115)

this does only get rectified by the next full folding which happens
in forwprop4 which is after the strlen pass emitting the unwanted
diagnostic.  The following mitigates this case in a similar way as
r15-7472 did for PR118817 - by ensuring we have the IL folded.
This is done by simply folding all immediate uses of the former
PHI def that SCCP replaces.  All other more general approaches have
too much fallout at this point.

	PR tree-optimization/118521
	* tree-scalar-evolution.cc (final_value_replacement_loop):
	Fold uses of the replaced PHI def.

	* g++.dg/torture/pr118521.C: New testcase.