CodeGen: Use a single SlotTracker in MachineFunction::print()

CodeGen: Use a single SlotTracker in MachineFunction::print()

Expose enough of the IR-level `SlotTracker` so that
`MachineFunction::print()` can use a single one for printing
`BasicBlock`s.  Next step would be to lift this through a few more APIs
so that we can make other print methods faster.

Fixes PR23865, changing the runtime of `llc -print-machineinstrs` from
many minutes (killed after 3 minutes, but it wasn't very close) to
13 seconds for a 502185 line dump.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240842 91177308-0d34-0410-b5e6-96231b3b80d8