[DAG] Move one-use add of splat to base of scatter/gather

This extends the uniform base transform used with scatter/gather to support one-use vector adds-of-splats with a non-zero base. This has the effect of essentially reassociating an add from vector to scalar domain.

The motivation is to improve the lowering of scatter/gather operations fed by complex geps.

Differential Revision: https://reviews.llvm.org/D134472