tree-optimization/108076 - if-conversion and forced labels

When doing if-conversion we simply throw away labels without checking
whether they are possibly targets of non-local gotos or have their
address taken.  The following rectifies this and refuses to if-convert
such loops.

	PR tree-optimization/108076
	* tree-if-conv.cc (if_convertible_loop_p_1): Reject blocks
	with non-local or forced labels that we later remove
	labels from.

	* gcc.dg/torture/pr108076.c: New testcase.