[MachineVerifier] Live interval for a subreg must have subranges

MachineVerifier verified the subranges of a live interval if
they existed, but did not complain if they did not exist.

This patch changes the verifier to complain if there are no
subranges in the live interval for a subreg operand (so long
as MachineRegisterInfo says we should be tracking subreg
liveness for that register). This matches the conditions for
LiveIntervalCalc to create subranges in the first place.

Differential Revision: https://reviews.llvm.org/D112556