Extend InstVisitor and Walker to handle arbitrary CFG functions, expand the

Function::walk functionality into f->walkInsts/Ops which allows visiting all
instructions, not just ops.  Eliminate Function::getBody() and
Function::getReturn() helpers which crash in CFG functions, and were only kept
around as a bridge.

This is step 25/n towards merging instructions and statements.

PiperOrigin-RevId: 227243966