- Replace 2 QHash<BasicBlock *, ...> with QVector<...>, where the
  basic-block index is the index of the vector.
- Nearly all QHashes and QSets will have a minimal fill rate. So,
  initialize/reserve all of them with a reasonable minimal size to
  prevent re-allocations and re-hashing.

Change-Id: Iade857991d73fddd0b92cecb8d458064b253a08d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>