Estimate a cost using the possible number of scratch registers required and use

Estimate a cost using the possible number of scratch registers required and use
it as a late BURR scheduling tie-breaker.
Intuitively, it's good to push down instructions whose results are liveout so
their long live ranges won't conflict with other values which are needed inside
the BB. Further prioritize liveout instructions by the number of operands which
are calculated within the BB.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35109 91177308-0d34-0410-b5e6-96231b3b80d8