[memdep] Just require domtree for memdep.

This doesn't cause us to construct dominator trees any more often in the
normal pipeline, and removes an entire mode of memdep that needed to be
reasoned about and maintained. Perhaps more importantly, it removes the
ability for the results of memdep to be different because of accidental
pass scheduling goofs or the order of evaluation of 'getResult' calls.

Essentially, 'getCachedResult', unless across IR-unit boundaries, is
extremely dangerous. We need to work much harder to avoid it (or its
analog in the old pass manager).

llvm-svn: 263232