Precompute some information about register costs.

Remember the minimum cost of the registers in an allocation order and
the number of registers at the end of the allocation order that have the
same cost per use.

This information can be used to limit the search space for
RAGreedy::tryEvict() when looking for a cheaper register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172280 91177308-0d34-0410-b5e6-96231b3b80d8