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
Kerry McLaughlin