[TBAA] Use wrapper objects instead of raw getOperand s; NFC

This is intended to make the semantic intent clearer.

The wrapper objects are now generic to avoid `const_cast` s.  Since
`const` ness is part of the API of `MDNode::getMostGenericTBAA` (and
therefore I can't make things `const` all the way through without some
code churn outside TypeBasedAliasAnalysis.cpp), this seemed like the
cleanest solution.

llvm-svn: 285665