mode-switching: Avoid quadractic list operation

mode-switching: Avoid quadractic list operation

add_seginfo chained insn information to the end of a list
by starting at the head of the list.  This patch avoids the
quadraticness by keeping track of the tail pointer.

gcc/
	* mode-switching.cc (add_seginfo): Replace head pointer with
	a pointer to the tail pointer.
	(optimize_mode_switching): Update calls accordingly.