tree-optimization/112310 - code hoisting undefined behavior

tree-optimization/112310 - code hoisting undefined behavior

The following avoids hoisting expressions that may invoke undefined
behavior and are not computed on all paths.  This is realized by
noting that we have to avoid materializing expressions as part
of hoisting that are not part of the set of expressions we have
found eligible for hoisting.  Instead of picking the expression
corresponding to the hoistable values from the first successor
we now keep a union of the expressions so that hoisting can pick
the expression that has its dependences fully hoistable.

	PR tree-optimization/112310
	* tree-ssa-pre.cc (do_hoist_insertion): Keep the union
	of expressions, validate dependences are contained within
	the hoistable set before hoisting.

	* gcc.dg/torture/pr112310.c: New testcase.