[VP] Correct LEGALPOS for more VP nodes.

LEGALPOS appears to only be used by LegalizeVectorOps. It needs
to point at a vector operand. Stores need to point at the second
operand since the result and the first operand are MVT::Other.
Reductions need to point at the second operand since the result
and the first operand are scalsrs.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D133048