[LegalizeVectorOps] Pass the post-UpdateNodeOperands version of Op to ExpandLoad/ExpandStore

UpdateNodeOperands might CSE to another existing node. So we should make sure we're legalizing that node otherwise we might fail to hook up the operands properly. I've moved the result registration up to the caller to avoid having to pass both Result and Op into the functions where it might be confusing which is which.

This address 2 other issues pointed out in D71861.

Differential Revision: https://reviews.llvm.org/D72021