[APInt] Fix nearestLogBase2 to return correct answers for very large APInt and APInt with a bitwidth of 1.

I also improved the comments, added some more tests, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199610 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed