Handle non-~0 lane masks on live-in registers in LivePhysRegs

When LivePhysRegs adds live-in registers, it recognizes ~0 as a special
lane mask indicating the entire register. If the lane mask is not ~0,
it will only add the subregisters that overlap the specified lane mask.

The problem is that if a live-in register does not have subregisters,
and the lane mask is not ~0, it will not be added to the live set.
(The given lane mask may simply be the lane mask of its register class.)

If a register does not have subregisters, add it to the live set if
the lane mask is non-zero.

Differential Revision: https://reviews.llvm.org/D26094

llvm-svn: 285440