optimizeGatherScatterInst does nothing specific to fixed length vectors but uses FixedVectorType to extract the number of elements. This patch simply updates the code to use VectorType and getElementCount instead. For testing I just copied Transforms/CodeGenPrepare/X86/gather-scatter-opt.ll replacing `<4 x ` with `<vscale x 4`. Differential Revision: https://reviews.llvm.org/D92572