Fix profile update in duplicat_loop_body_to_header_edge for loops with 0 count_in

this patch makes duplicate_loop_body_to_header_edge to not drop profile counts to
uninitialized when count_in is 0.  This happens because profile_probability in 0 count
is undefined.

gcc/ChangeLog:

	* cfgloopmanip.cc (duplicate_loop_body_to_header_edge): Special case loops with
	0 iteration count.