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.
