middle-end/94614 - avoid multiword moves to nothing

middle-end/94614 - avoid multiword moves to nothing

This adjusts emit_move_multi_word to handle moves into paradoxical
subregs parts that are not there and adjusts lower-subregs
CLOBBER resolving to deal with those as well.

2020-04-16  Richard Biener  <rguenther@suse.de>

	PR middle-end/94614
	* expr.c (emit_move_multi_word): Do not generate code when
	the destination part is undefined_operand_subword_p.
	* lower-subreg.c (resolve_clobber): Look through a paradoxica
	subreg.