[SelectionDAG] Remove some code from PromoteIntOp_MGATHER that handles UpdateNodeOper...

[SelectionDAG] Remove some code from PromoteIntOp_MGATHER that handles UpdateNodeOperands returning an existing node instead of updating.

I suspect this became unecessary when the CSE of mgather was fixed in r338080. It may still be possible to hit this if we widen the element type of a gather outside of type legalization and the promote the mask of a separate gather node so they become the same. But that seems pretty unlikely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342022 91177308-0d34-0410-b5e6-96231b3b80d8