[InstSimplify] Return poison for out of bounds extractelement

[InstSimplify] Return poison for out of bounds extractelement

This is the same change as D93990, but for extractelement rather
than insertelement.

> If idx exceeds the length of val for a fixed-length vector, the
> result is a poison value. For a scalable vector, if the value of
> idx exceeds the runtime length of the vector, the result is a
> poison value.