ART: Fix single-step interpretation for mterp

For bring-up (and failover) purposes, mterp can bail out to the
switch interpreter to handle a single opcode.  This CL fixes the
handoff of the result register (a bug that was masked by an earlier
revision which passed it around by reference, rather than by value.

Change-Id: Iba9e1576f52b3660348c89466438bdc0526227d0