MachineVerifier: Handle the optional def operand in a PATCHPOINT instruction.

The PATCHPOINT instructions have a single optional defined register operand,
but the machine verifier can't verify the optional defined register operands.
This commit makes sure that the machine verifier won't report an error when a
PATCHPOINT instruction doesn't have its optional defined register operand.
This change will allow us to enable the machine verifier for the code
generation tests for the patchpoint intrinsics.

Reviewers: Juergen Ributzka
llvm-svn: 244513