PR rtl-optimization/51933
	* ree.c (transform_ifelse): Return true right away if dstreg is
	already wider or equal to cand->mode.
	(enum ext_modified_kind, struct ext_modified, ext_state): New types.
	(make_defs_and_copies_lists): Remove defs_list and copies_list
	arguments, add state argument, just truncate state->work_list
	instead of always allocating and freeing the vector.  Assert that
	get_defs succeeds instead of returning 2.  Changed return type to
	bool.
	(merge_def_and_ext): Add state argument.  If SET_DEST doesn't
	have ext_src_mode, see if it has been modified already with the
	right kind of extension and has been extended before from the
	ext_src_mode.  If SET_DEST is already wider or equal to cand->mode,
	just return true.  Remember the original mode in state->modified
	array.
	(combine_reaching_defs): Add state argument.  Don't allocate and
	free here def_list, copied_list and vec vectors, instead just
	VEC_truncate the vectors in *state.  Don't handle outcome == 2
	here.
	(find_and_remove_re): Set DF_DEFER_INSN_RESCAN df flag.
	Add state variable, clear vectors in it, initialize state.modified
	if needed.  Free all the vectors at the end and state.modified too.
	Don't skip a candidate if the extension expression has been modified.

	* gcc.c-torture/execute/pr51933.c: New test.

From-SVN: r183416