[SCEV] List all binops in getOperandsToCreate()

Explicitly list all binops rather than having a default case. There
were two bugs here:
1. U->getOpcode() was used instead of BO->Opcode, which means we
   used the logic for the wrong opcode in some cases.
2. SCEV construction does not support LShr. We should return
   unknown for it rather than recursing into the operands.