Split LiveRangeCalc in LiveRangeCalc/LiveIntervalCalc. NFC

Summary:
Refactor LiveRangeCalc such that it is now split into two classes

The objective is to split all the "register specific" logic away
from LiveRangeCalc.
The two new classes created are:

- LiveRangeCalc - is meant as a generic class to compute and modify
  live ranges in a generic way. This class should deal only with
  SlotIndices and VNInfo objects.

- LiveIntervalCals - is meant to be equivalent to the old LiveRangeCalc.
  It computes the liveness virtual registers tracked by a LiveInterval
  object.

With this refactoring LiveRangeCalc can be used to implement tracking of
liveness of LiveRanges that represent other things than just registers.

Subscribers: MatzeB, qcolombet, mgorny, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76584