rtl-ssa: Fix handling of deleted insns

RTL-SSA queues up some invasive changes for later.  But sometimes
the insns involved in those changes can be deleted by later
optimisations, making the queued change unnecessary.  This patch
checks for that case.

gcc/
	* rtl-ssa/changes.cc (function_info::perform_pending_updates): Check
	whether an insn has been replaced by a note.