[RISCV] Remove 'frameindex' from list for ComplexPattern. NFC

Putting a node in this list allows the node to be used as the root
of an isel pattern that would then call the ComplexPattern. The
usual case is to use the ComplexPattern as the operand of another
operator.

AddrFI is never used as a root operation. frameindex is handled
directly with custom code in RISCVISelDAGToDAG::Select. So adding
frameindex to the list here serves no purpose.