CodeGen: Bring back MachineBasicBlock::iterator::getInstrIterator()...

This is a little embarrassing.

When I reverted r261504 (getIterator() => getInstrIterator()) in
r261567, I did a `git grep` to see if there were new calls to
`getInstrIterator()` that I needed to migrate.  There were 10-20 hits,
and I blindly did a `sed ...` before calling `ninja check`.

However, these were `MachineInstrBundleIterator::getInstrIterator()`,
which predated r261567.  Perhaps coincidentally, these had an identical
name and return type.

This commit undoes my careless sed and restores
`MachineBasicBlock::iterator::getInstrIterator()`.

llvm-svn: 261577