Fix -widen-vmovs liveness issues.

When widening a copy, we are reading a larger register that may not be
live.  Use an <undef> flag to tell the register scavenger and machine
code verifier that we know the value isn't defined.

We now widen:

  %S6<def> = COPY %S4<kill>, %D3<imp-def>

into:

  %D3<def> = VMOVD %D2<undef>, pred:14, pred:%noreg, %S4<imp-use,kill>

This also keeps the <kill> flag on %S4 so we don't inadvertently kill a
live value in %S5.

Finally, ensure that ARMBaseInstrInfo::setExecutionDomain() preserves
the <undef> flag when converting VMOVD to VORR.

llvm-svn: 141746