[RISCV] Use getVectorIdxConstant in RISCVISelLowering.cpp. NFC (#83019)

We use getVectorIdxConstant() in some places and getConstant(XLenVT) or
getIntPtrConstant() in others, but getVectorIdxTy() == getPointerTy() ==
XLenVT.

This refactors RISCVISelLowering to use the former for nodes that use
getVectorIdxTy(), i.e. INSERT_SUBVECTOR, EXTRACT_SUBVECTOR,
INSERT_VECTOR_ELT and EXTRACT_VECTOR_ELT, so that we're consistent.