rtl-ssa: Fix bug in function_info::add_insn_after

In the case that !insn->is_debug_insn () && next->is_debug_insn (), this
function was missing an update of the prev pointer on the first nondebug
insn following the sequence of debug insns starting at next.

This can lead to corruption of the insn chain, in that we end up with:

  insn->next_any_insn ()->prev_any_insn () != insn

in this case.  This patch fixes that.

gcc/ChangeLog:

	* rtl-ssa/insns.cc (function_info::add_insn_after): Ensure we
	update the prev pointer on the following nondebug insn in the
	case that !insn->is_debug_insn () && next->is_debug_insn ().