[AMDGPU] Do not insert an instruction into worklist twice in movetovalu

[AMDGPU] Do not insert an instruction into worklist twice in movetovalu

In moveToVALU(), move to vector ALU is performed, all instrs in
the use chain will be visited. We do not want the same node to be
pushed to the visit worklist more than once.

Differential Revision: https://reviews.llvm.org/D34726

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308039 91177308-0d34-0410-b5e6-96231b3b80d8