AMDGPU: Take care of "tied" operand when removeOperand

Summary:
  Flat scratch load of D16 type by default has tied vdst_in operand (with vdst). This should be taken
care of at the time of "removeOperand" in eliminateFrameIndex. Otherwise we will hit an assert saying
"Cannot move tied operands". This patch unties vdst_in before the move, and retie it with vdst afterwards.

Reviewers:
  arsenm, foad

Differential Revision: https://reviews.llvm.org/D130537