Reimplement the internal abstraction used by MemDep in terms

Reimplement the internal abstraction used by MemDep in terms
of a pointer/int pair instead of a manually bitmangled pointer.
This forces clients to think a little more about checking the
appropriate pieces and will be useful for internal
implementation improvements later.

I'm not particularly happy with this.  After going through this
I don't think that the clients of memdep should be exposed to
the internal type at all.  I'll fix this in a subsequent commit.

This has no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60230 91177308-0d34-0410-b5e6-96231b3b80d8