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

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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256789 91177308-0d34-0410-b5e6-96231b3b80d8