i386: Use offsetable address constraint for double-word memory operands

Double-word memory operands are accessed as their high and low part, so the
memory location has to be offsettable.  Use "o" constraint instead of "m"
for double-word memory operands.

gcc/ChangeLog:

	* config/i386/i386.md (*insvdi_lowpart_1): Use "o" constraint
	instead of "m" for double-word mode memory operands.
	(*add<dwi>3_doubleword_zext): Ditto.
	(*addv<dwi>4_doubleword_1): Use "jO" constraint instead of "jM"
	for double-word mode memory operands.