[CodeGenPrepare] Update optimizeGatherScatterInst for scalable vectors.

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