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[LoopVectorize] Use MapVector rather than DenseMap for MinBWs.

Description

[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
interesting.

Many thanks to David Blaikie for making me aware of MapVector!

Fixes PR25490.

Differential Revision: http://reviews.llvm.org/D14981

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