BasicBlocks have an index property which points to the index of that
basic block in the container array in Function. This property can be
used to store calculated information about basic blocks in a vector,
where the vector index corresponds to the basic block index. This is
a lot cheaper than storing any information in a
QHash<BasicBlock *, ....>.

However, this numbering requires that no re-ordering or deletion of
blocks happens. This change cleans up all that handling which was
scattered over a number of places.

Change-Id: I337abd39c030b9d30c82b7bbcf2ba89e50a08e63
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>