change_zero_ext handles (zero_extend:M1 (subreg:M2 (reg:M1) ...))
already; this patch extends it to also deal with any
(zero_extend:M1 (subreg:M2 (reg:M3) ...)) where the subreg is not
paradoxical.


	* combine.c (change_zero_ext): Also handle extends from a subreg
	to a mode bigger than that of the operand of the subreg.

From-SVN: r242950