[ARM][ReachingDefs] RDA in LoLoops

Add several new methods to ReachingDefAnalysis:
- getReachingMIDef, instead of returning an integer, return the
  MachineInstr that produces the def.
- getInstFromId, return a MachineInstr for which the given integer
  corresponds to.
- hasSameReachingDef, return whether two MachineInstr use the same
  def of a register.
- isRegUsedAfter, return whether a register is used after a given
  MachineInstr.

These methods have been used in ARMLowOverhead to replace searching
for uses/defs.

Differential Revision: https://reviews.llvm.org/D70009