[LiveIntervals] Replace handleMoveIntoBundle

[LiveIntervals] Replace handleMoveIntoBundle

Summary:
The current handleMoveIntoBundle implementation is unusable,
it attempts to access the slot indexes of bundled instructions.
It also leaves bundled instructions with slot indexes assigned.

Replace handleMoveIntoBundle this with a more explicit
handleMoveIntoNewBundle function which recalculates the live
intervals for all instructions moved into a newly formed bundle,
and removes slot indexes from these instructions.

Reviewers: arsenm, MaskRay, kariddi, tpr, qcolombet

Reviewed By: qcolombet

Subscribers: MatzeB, wdng, hiraditya, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77969