MachineVerifier: Improve register operand checks

This fixes cases where we wouldn't perform various register operand
checks just because we didn't happen to have a definition in the
MCInstrDesc. This changes the code to only skip the tests that actually
depend on the MCInstrDesc definition.

This makes the machine verifier spot the problem from
https://llvm.org/PR33071 after the pass that actually caused it.

llvm-svn: 319141