V4 IR: change BasicBlockSet::const_iterator to use std::find.

When the storage in the BasicBlockSet is a std::vector<bool>, the
begin() iterator and the ++operator now use std::find. Good STL
implementations have an optimized version specifically for
std::vector<bool> that is a lot faster than the iterating that was used
before. The change also makes the code more readable.

Change-Id: Ic37bac694c133c597b3d61a91b04a0b758516dc3
Reviewed-by: Lars Knoll <lars.knoll@digia.com>