Add getScaledIntervalSize, which gives a measure of the size of an interval that...

Add getScaledIntervalSize, which gives a measure of the size of an interval that is independent of the scaling of
the function due to empty index slots.  This is suitable for use in backend heuristics that need to reason about the density
of an interval.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52652 91177308-0d34-0410-b5e6-96231b3b80d8