[SVE] Make ConstantFoldGetElementPtr work for scalable vectors of indices

This patch fixes a compiler crash that was hit when trying to simplify
the following code:

getelementptr [2 x i64], [2 x i64]* null, i64 0, <vscale x 2 x i64> zeroinitializer

For the case where we have a null pointer value like above, we just
need to ensure we don't assume the indices are always fixed width.

Differential Revision: https://reviews.llvm.org/D82183