[CodeGen] Fix unconditional branch duplication issue in bbsections

If an end section basic block ends in an unconditional branch to its
fallthrough, BasicBlockSections will duplicate the unconditional branch.
This doesn't break x86, but it is a (slight) size optimization and more
importantly prevents AArch64 builds from breaking.

Ex:
```
bb1 (bbsections Hot):
  jmp bb2

bb2 (bbsections Cold):
  /* do work... */
```

After running sortBasicBlocksAndUpdateBranches():
```
bb1 (bbsections Hot):
  jmp bb2
  jmp bb2

bb2 (bbsections Cold):
  /* do work... */
```

Differential Revision: https://reviews.llvm.org/D158674