[LoopVectorize] Use MapVector rather than DenseMap for MinBWs.
The order in which instructions are truncated in truncateToMinimalBitwidths
effects code generation. Switch to a map with a determinisic order, since the
iteration order over a DenseMap is not defined.
This code is not hot, so the difference in container performance isn't
Many thanks to David Blaikie for making me aware of MapVector!
Differential Revision: http://reviews.llvm.org/D14981