[SVE][CodeGen] Vector + immediate addressing mode for masked gather/scatter

This patch extends LowerMGATHER/MSCATTER to make use of the vector + reg/immediate
addressing modes for scalable masked gathers & scatters.

selectGatherScatterAddrMode checks if the base pointer is null, in which case
we can swap the base pointer and the index, e.g.
     getelementptr nullptr, <vscale x N x T> (splat(%offset)) + %indices)
  -> getelementptr %offset, <vscale x N x T> %indices

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D93132