tree-optimization/107690 - avoid creating un-analyzable loop exits

tree-optimization/107690 - avoid creating un-analyzable loop exits

The following adds a heuristic to ifcombine that avoids turning
analyzable loop exits into unanalyzable ones.  This allows vectorizing
the testcase in the PR again.  I've refrained from actually
analyzing niters but instead used a cheaper heuristic.  I believe
we'll only ever attempt to combine two ifs if they are in the same
loop and if either both exit the loop or stay within.

	PR tree-optimization/107690
	* tree-ssa-ifcombine.cc (ifcombine_ifandif): Do not merge
	possibly analyzable exit conditions.

	* g++.dg/vect/vect-pr107690.cc: New testcase.