[MachineVerifier] Fix crash on early clobbered subreg operands.

MachineVerifier tried to checkLivenessAtDef() ignoring it is actually a subreg.

The issue was with processing two subregs of the same reg are used in the same
instruction (e.g. inline asm): "def early-clobber" and other just "def".

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D126661