tree-optimization/107852 - missed optimization with PHIs

tree-optimization/107852 - missed optimization with PHIs

The following deals with the situation where we have

<bb 2> [local count: 1073741824]:
_5 = bytes.D.25336._M_impl.D.24643._M_start;
_6 = bytes.D.25336._M_impl.D.24643._M_finish;
pretmp_66 = bytes.D.25336._M_impl.D.24643._M_end_of_storage;
if (_5 != _6)
  goto <bb 3>; [70.00%]
else
  goto <bb 4>; [30.00%]

...

<bb 6> [local count: 329045359]:
_89 = operator new (4);
_43 = bytes.D.25336._M_impl.D.24643._M_start;
_Num_44 = _137 - _43;
if (_Num_44 != 0)

but fail to see that _137 is equal to _5 and thus eventually _Num_44
is zero if not operator new would possibly clobber the global
bytes variable.

The following resolves this in value-numbering by using the
predicated values for _5 == _6 recorded for the dominating
condition.

	PR tree-optimization/107852
	* tree-ssa-sccvn.cc (visit_phi): Use equivalences recorded
	as predicated values to elide more redundant PHIs.

	* gcc.dg/tree-ssa/ssa-fre-101.c: New testcase.