* lower-subreg.c (subreg_context): New static bitmap.
	(decompose_multiword_subregs): Allocate and free it.
	(find_decomposable_subregs): Set a bit in it for a register that
	occurs in a subreg that changes mode but not size.
	(can_decompose_p): Test it instead of non_decomposable_context.

From-SVN: r162181