[SelectionDAGBuilder] Don't set MachinePointerInfo for gather when we find a uniform base

I believe we were previously calculating a pointer info with the scalar base and an offset of 0. But that's not really where the gather is pointing. The offset is a function of the indices of the GEP we looked through.

Also set the size of the MachineMemOperand to UnknownSize

Differential Revision: https://reviews.llvm.org/D76157