tree-optimization/105646 - re-interpret always executed in uninit diag

tree-optimization/105646 - re-interpret always executed in uninit diag

The following fixes PR105646, not diagnosing

int f1();
int f3(){
    auto const & a = f1();
    bool v3{v3};
    return a;
}

with optimization because the early uninit diagnostic pass only
diagnoses always executed cases.  The patch does this by
re-interpreting what always executed means and choosing to
ignore exceptional and abnormal control flow for this.  At the
same time it improves things as suggested in a comment - when
the value-numbering run done without optimizing figures there's
a fallthru path, consider blocks on it as always executed.

	PR tree-optimization/105646
	* tree-ssa-uninit.cc (warn_uninitialized_vars): Pre-compute
	the set of fallthru reachable blocks from function entry
	and use that to determine wlims.always_executed.

	* g++.dg/uninit-pr105646.C: New testcase.