[IR] fix potential crash in Constant::isElementWiseEqual()

There's only one user of this API currently, and it seems
impossible that it would compare values with different types.

But that's not true in general, so we need to make sure the
types are the same.

As denoted by the FIXME comments, we will also crash on FP
values. That's what brought me here, but we can make that a
follow-up patch.