[NFC][RISCV] Move calculations of SDNode policy operand idx to a separate function

Since there is no guaranteed correspondence of SDNode and MI operands, we need
getters simular to RISCVII::get*OpNum for SDNodes.

More uses of getVecPolicyOpIdx will be added in D130895.

Reviewed By: craig.topper, arcbbb

Differential Revision: https://reviews.llvm.org/D134179