Simplify MachineVerifier's block-successor verification.

There's two properties we want to verify:

1. That the successors returned by analyzeBranch are in the CFG
   successor list, and
2. That there are no extraneous successors are in the CFG successor
   list.

The previous implementation mostly accomplished this, but in a very
convoluted manner.

Differential Revision: https://reviews.llvm.org/D79793