[RegisterCoalescer] Account for instructions deleted by removePartialredunduncy and in WorkList

Summary:
 removePartialRedundency optimization introduces a state in the
 RegisterCoalescer where an instruction pointed to in the WorkList
 is deleted from the MBB and then removed from the ErasedList.
 This patch updates the ErasedList to be used globally by not erasing
 erased Instructions from  it to solve the problem.

 The patch also accounts for the case where an Instruction was previously
 deleted and the same memory was reused by BuildMI to create a new instruction.

Reviewers: kparzysz, qcolombet

Reviewed By: qcolombet

Subscribers: MatzeB, qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D34902

llvm-svn: 306915