CodeGen: Split bundle_iterator into a separate file, NFC

Split MachineBasicBlock::bundle_iterator into a separate file, and
rename the class to MachineBundleIterator.

This is a precursor to adding a `MachineInstr::getBundleIterator()`
accessor, which will eventually let us delete the final call to
getNodePtrUnchecked(), and then remove the UB from ilist_iterator.

As a drive-by, I removed an unnecessary second template parameter.

llvm-svn: 261502