[VP] Correct the LEGALPOS for VP_STORE.

VP_STORE has a Chain for operand 0, so the LEGALPOS should be 1.

VP_STORE is always considered Legal for MVT::Other. So I suspect this
was causing vp_store to be ignored by LegalizeVectorOps and instead
handled in LegalizeDAG.

VP_LOAD is Custom expanded in LegalizeVectorOps for RISC-V.

Differential Revision: https://reviews.llvm.org/D132972