[FIX] Propagate execution domain of invariant loads

  If the base pointer of an invariant load is is loaded conditionally, that
  condition needs to hold for the invariant load too. The structure of the
  program will imply this for domain constraints but not for imprecisions in
  the modeling. To this end we will propagate the execution context of base
  pointers during code generation and thus ensure the derived pointer does
  not access an invalid base pointer.

llvm-svn: 267707