Remove the experimental AliasAnalysis::getDependency interface, which

isn't a good level of abstraction for memdep. Instead, generalize
AliasAnalysis::alias and related interfaces with a new Location
class for describing a memory location. For now, this is the same
Pointer and Size as before, plus an additional field for a TBAA tag.

Also, introduce a fixed MD_tbaa metadata tag kind.

llvm-svn: 113858