ART: Replace expensive calls to Covers in reg alloc

LiveInterval::Covers is implemented as a linear-time search over
liveness ranges and can therefore be rather expensive and should be
avoided unless necessary. This patch replaces calls to Covers when
searching for a sibling with the cheaper IsDefinedAt call.

Change-Id: I93fc73529c15a518335f4cbdc3a0def52d9501e5