Auxiliary functions to determine the set of memory accesses which can't trap because they are preceded by accesses to the same memory portion. We do that for MEM_REFs, so we only need to track the SSA_NAME of the pointer indirectly referenced. The algorithm simply is a walk over all instructions in dominator order. When we see an MEM_REF we determine if we've already seen a same ref anywhere up to the root of the dominator tree. If we do the current access can't trap. If we don't see any dominating access the current access might trap, but might also make later accesses non-trapping, so we remember it. We need to be careful with loads or stores, for instance a load might not trap, while a store would, so if we see a dominating read access this doesn't mean that a later write access would not trap. Hence we also need to differentiate the type of access(es) seen.
??? We currently are very conservative and assume that a load might trap even if a store doesn't (write-only memory). This probably is overly conservative. A hash-table of SSA_NAMEs, and in which basic block an MEM_REF through it was seen, which would constitute a no-trap region for same accesses.
