Use canonical forms for the branch probability zero heutistic.

- Drop support for X >u 0, it's equivalent to X != 0 and should be canonicalized into the latter.
- Add X < 1 -> unlikely, which is what instcombine canonicalizes X <= 0 into.
- Add X > -1 -> likely, which is what instcombine canonicalizes X >= 0 into.

llvm-svn: 139110