[DivRemPairs] Fix non-determinism in use list order.

Summary:
Use a MapVector instead of a DenseMap for RemMap since it is iteratated
over and the order of iteration can effect the order that new
instructions are created.  This can in turn effect the use list order of
div/rem input values if multiple new instructions are created that share
any input values.

Reviewers: spatel

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D45858

llvm-svn: 330792