[PATCH, MIPS] Use dext in zero_extend

[Adam Nemet]

Richard Sandiford writes:
> OK, thanks.

Thanks.

> For the record more than anything: I did wonder whether we should
> have something more specific than "arith" here, but I agree it's
> better not to.  The policy has generally been to lump things together
> until we find we need to split them.

Right, first I added zeroext but then I saw that [d]ext is arith everywhere
else.

Adam