[RegisterCoalescer] Add wrapper for Erasing Instructions

Summary:
      To delete an instruction the coalescer needs to call eraseFromParent()
      on the MachineInstr, insert it in the ErasedInstrs list and update the
      Live Ranges structure. This patch re-factors the code to do all that in
      one function. This will also fix cases where previous code wasn't
      inserting deleted instructions in the ErasedList.

Reviewers: qcolombet, kparzysz

Reviewed By: qcolombet

Subscribers: MatzeB, llvm-commits, qcolombet

Differential Revision: https://reviews.llvm.org/D36204

llvm-svn: 309915