[llvm][CodeGen] Do not scalarize `llvm.masked.[gather|scatter]` operating on scalable vectors.

This patch prevents the `llvm.masked.gather` and `llvm.masked.scatter` intrinsics to be scalarized when invoked on scalable vectors.

The change in `Function.cpp` is needed to prevent the warning that is raised when `getNumElements` is used in place of `getElementCount` on `VectorType` instances. The tests guards for regressions on this change.

The tests makes sure that calls to `llvm.masked.[gather|scatter]` are still scalarized when:

  # the intrinsics are operating on fixed size vectors, and
  # the compiler is not targeting fixed length SVE code generation.

Reviewed By: efriedma, sdesmalen

Differential Revision: https://reviews.llvm.org/D86249