revamp BoundsChecking considerably:

 - compute size & offset at the same time. The side-effects of this are that we now support negative GEPs. It's now approaching a phase that it can be reused by other passes (e.g., lowering of the objectsize intrinsic)
 - use APInt throughout to handle wrap-arounds
 - add support for PHI instrumentation
 - add a cache (required for recursive PHIs anyway)
 - remove hoisting support for now, since it was wrong in a few cases

sorry for the churn here.. tests will follow soon.

llvm-svn: 157775