[FIX] Require base pointers of loads that might alias to be hoisted

  Since the base pointer of a possibly aliasing pointer might not alias
  with any other pointer it (the base pointer) might not be tagged as
  "required invariant". However, we need it do be in order to compare
  the accessed addresses of the derived (possibly aliasing) pointer.

  This patch also tries to clean up the load hoisting a little bit.

llvm-svn: 270412