[RISCV][TableGen] Correct vTtoGetLlvmTyString for RISC-V tuples. (#162152)
RISC-V tuples use "NF" not "nElem" to store the number of fields in the
segment.
This fixes a crash when lowering a function with tuple return.
getReturnInfo in CallLowering.cpp does Type*->EVT->Type* and we were
incorrectly converting EVT to Type*.