[ADT] Replace the logb implementation with the simpler and much closer
to what we actually want ilogb implementation. This makes everything
*much* easier to deal with and is actually what we want when using it
anyways.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219474 91177308-0d34-0410-b5e6-96231b3b80d8