A small refactoring + adding comments.

SelectionDAGIsel::LowerArguments needs a function, not a basic block. So it
makes sense to pass it the function instead of extracting a basic-block from
the function and then tossing it. This is also more self-documenting (functions
have arguments, BBs don't).

In addition, added comments to a couple of Select* methods.

llvm-svn: 176305