c++: Relax too strict assert in stabilize_expr [PR111160]

c++: Relax too strict assert in stabilize_expr [PR111160]

The case in the ticket is an ICE on invalid due to an assert in stabilize_expr,
but the underlying issue can actually trigger on this *valid* code:

=== cut here ===
struct TheClass {
  TheClass() {}
  TheClass(volatile TheClass& t) {}
  TheClass operator=(volatile TheClass& t) volatile { return t; }
};
void the_func() {
  volatile TheClass x, y, z;
  (false ? x : y) = z;
}
=== cut here ===

The problem is that stabilize_expr asserts that it returns an expression
without TREE_SIDE_EFFECTS, which can't be if the involved type is volatile.

This patch relaxes the assert to accept having TREE_THIS_VOLATILE on the
returned expression.

Successfully tested on x86_64-pc-linux-gnu.

	PR c++/111160

gcc/cp/ChangeLog:

	* tree.cc (stabilize_expr): Stabilized expressions can have
	TREE_SIDE_EFFECTS if they're volatile.

gcc/testsuite/ChangeLog:

	* g++.dg/overload/error8.C: New test.
	* g++.dg/overload/volatile2.C: New test.