LiveIntervalAnalysis: Avoid multiple connected liveness components

We may have subregister defs which are unused but not discovered and
cleaned up prior to liveness analysis. This creates multiple connected
components in the resulting live range which are forbidden in the
MachineVerifier because they would unnecesarily constrain the register
allocator. Rewrite those dead definitions to define a newly created
virtual register.

Differential Revision: http://reviews.llvm.org/D13035

llvm-svn: 248335