[PR68083] don't introduce undefined behavior in ifcombine

[PR68083] don't introduce undefined behavior in ifcombine

The ifcombine pass may move a conditional access to an uninitialized
value before the condition that ensures it is always well-defined,
thus introducing undefined behavior.  Stop it from doing so.

for  gcc/ChangeLog

	PR tree-optimization/68083
	* tree-ssa-ifcombine.c: Include tree-ssa.h.
	(bb_no_side_effects_p): Test for undefined uses too.
	* tree-ssa.c (gimple_uses_undefined_value_p): New.
	* tree-ssa.h (gimple_uses_undefined_value_p): Declare.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/68083
	* gcc.dg/torture/pr68083.c: New.  From Zhendong Su.

From-SVN: r229690