tree-optimization/111773 - avoid CD-DCE of noreturn special calls

tree-optimization/111773 - avoid CD-DCE of noreturn special calls

The support to elide calls to allocation functions in DCE runs into
the issue that when implementations are discovered noreturn we end
up DCEing the calls anyway, leaving blocks without termination and
without outgoing edges which is both invalid IL and wrong-code when
as in the example the noreturn call would throw.  The following
avoids taking advantage of both noreturn and the ability to elide
allocation at the same time.

For the testcase it's valid to throw or return 10 by eliding the
allocation.  But we have to do either where currently we'd run
off the function.

	PR tree-optimization/111773
	* tree-ssa-dce.cc (mark_stmt_if_obviously_necessary): Do
	not elide noreturn calls that are reflected to the IL.

	* g++.dg/torture/pr111773.C: New testcase.