[llvm][ADT] Remove duplicate code in llvm::StringMapImpl::RehashTable

Remove duplicate code in llvm::StringMapImpl::RehashTable, near StringMap.cpp:229

```
if (!NewTableArray[NewBucket]) {
  NewTableArray[FullHash & (NewSize - 1)] = Bucket;
  NewHashArray[FullHash & (NewSize - 1)] = FullHash;
  if (I == BucketNo)
    NewBucketNo = NewBucket;
  continue;
}
```

Reviewed By: MaskRay, dexonsmith

Differential Revision: https://reviews.llvm.org/D121726