[CodeGen] Use the TII hook for Noop insertion in BBSections (NFC)

Refactor BasicBlockSections to use the target-specific noop insertion
hook from TargetInstrInfo instead of building it ourselves. Using the
TII hook is both cleaner and makes it easier to extend BBSections to
non-X86 targets.

Differential Revision: https://reviews.llvm.org/D158303