Clarify that the bypassSlowDivision optimization operates on a single BB [v2]

Update some comments to be more explicit.

Change bypassSlowDivision and the functions it calls so that they take
BasicBlock*s and Instruction*s, rather than Function::iterator&s and
BasicBlock::iterator&s.

Change the APIs so that the caller is responsible for updating the
iterator, rather than the callee. This makes control flow much easier
to follow.

Patch by Justin Lebar!

llvm-svn: 256789