[LV] Remove nondeterminacy by changing LoopVectorizationLegality::Reductions

from DenseMap to MapVector

The iteration order of LoopVectorizationLegality::Reductions matters for the
final code generation, so we better use MapVector instead of DenseMap for it
to remove the nondeterminacy. reduction-order.ll in the patch is an example
reduced from the case we saw. In the output of opt command, the order of the
select instructions in the vector.body block keeps changing from run to run
currently.

Differential Revision: https://reviews.llvm.org/D73490