[InstSimplify] fold extractelement of splat with variable extract index
We already have a fold for variable index with constant vector,
but if we can determine a scalar splat value, then it does not
matter whether that value is constant or not.
We overlooked this fold in D102404 and earlier patches,
but the fixed vector variant is shown in:
https://llvm.org/PR50817
Alive2 agrees on that:
https://alive2.llvm.org/ce/z/HpijPC
The same logic applies to scalable vectors.
Differential Revision: https://reviews.llvm.org/D104867