Tweaks to the icmp+sext-to-shifts optimization to address Frits' comments:

- Localize the check if an icmp has one use to a place where we know we're
  introducing something that's likely more expensive than a sext from i1.
- Add an assert to make sure a case that would lead to a miscompilation is
  folded away earlier.
- Fix a typo.

llvm-svn: 128744