[InstCombine] limit pair-of-insertelement folds to avoid miscompile

This transform was added with 4446f71c. However, as noted in
the post-commit feedback, the transform is not safe with an
arbitrary base vector because we may leak poison from a narrow
element into an adjacent element when bitcasting.

I made the least invasive code change in case we do figure out
a way to make this safe.