Clarify HowFarToZero computation when the step is a positive power of two.  Functionally this should be identical to the existing code except for the case where Step is maximally negative (eg, INT_MIN).  We now punt in that one corner case to make reasoning about the code easier.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224274 91177308-0d34-0410-b5e6-96231b3b80d8