[LSV] Use OrderedBasicBlock instead of rolling it ourselves. NFC

Summary:
In getVectorizablePrefix, this is less efficient (because we have to
iterate over the BB twice), but boy is it simpler.  Given how much
trouble we've had here, I think the simplicity gain is worthwhile.

In reorder(), this is actually more efficient, as
DominatorTree::dominates iterates over the BB from the beginning when
the two instructions are in the same BB.

Reviewers: asbirlea

Subscribers: arsenm, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D23472

llvm-svn: 278580