[MemDepAnalysis] Allow caller to pass in an OrderedBasicBlock.

If the caller can preserve the OBB, we can avoid recomputing the order
for each getDependency call.

Reviewers: efriedma, rnk, hfinkel

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D59788

llvm-svn: 357206