re PR rtl-optimization/32300 (ICE with -O2 -fsee)

	PR rtl-optimization/32300
	* see.c (see_copy_insn): New function.
	(see_def_extension_not_merged, see_merge_one_use_extension,
	see_merge_one_def_extension): Use it.  Avoid changing
	PREV_INSN/NEXT_INSN chains directly, insted emit insns
	into sequences.  Call df_insn_delete on temporary insns
	that won't be emitted into the insn stream.
	(rest_of_handle_see): Turn off DF_DEFER_INSN_RESCAN
	and run df_process_deferred_rescans () before run_fast_dce.

From-SVN: r128108