[AA] Make BasicAA just require domtree.

This doesn't change how many times we construct domtrees in the normal
pipeline, and it removes fragility and instability where basic-aa may
not be run in time to see domtrees because they happen to be constructed
afterward.

This isn't quite as clean as the change to memdep because there is
a mode where basic-aa specifically runs without domtrees -- in the
hacking version used by function-attrs with the legacy pass manager.

llvm-svn: 263234