re PR target/12712 (ICE on short legit C++ code fragment with gcc 3.3.2)

re PR target/12712 (ICE on short legit C++ code fragment with gcc 3.3.2)

	PR target/12712
	* reg-stack.c (convert_regs_1): Create an arbitrary input stack
	if the block has no predecessors.
	(convert_regs_2): Document the problem with successors whose
	only predecessor is the block to be processed.
	(convert_regs): Don't create the arbitrary input stack here.

From-SVN: r72923