MachineVerifier: Improve PHI operand checking

Additional checks for phi operands:
- first operand should be a virtual register def. It should not be
  tied, implicit, internalread, earlyclobber or a read.
- The other operands should be register/mbb operands next to each other
- The register operands should not be implicit, internalread,
  earlyclobber, debug or tied.
- We can perform most of the PHI checks even for unreachable blocks.

llvm-svn: 319140