Pointer validity checks are applied inconsistently

To check the validity of a BoundedPointer we have 3 methods; checkAlive for liveness checks, checkBounds for bounds checks, and checkAccess which call checkAlive, checks whether the given access mode is valid (i.e. read/write), and then does it's own version of a bounds check, instead of calling checkBounds.

As a result of this, some places only call one check or the other. We should clean this up, and make sure checks are applied consistently.