ART: Fix x86_64 GenSelect case when destination is Ref

ART: Fix x86_64 GenSelect case when destination is Ref

Reference in x86_64 is a 64-bit solo register. As a result, the invocation
  of OpRegImm results in an error when Select opcode of the kind:
    ref = boolean ? null : null;
because opRegImm does not support 64-bit destination for OpMov.

The case above is only possible for ref because no one other constant except
  null is possible.

Bug: 17327895
Change-Id: I7541e744ec1c8619711712fd17be72764efcf3a8
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>