[JumpThreading] Change asserts for WantInteger into actual checks

After e734e828, it is possible to end up in
a situation where an `indirectbr` is fed by a cast, which is in turn fed by
an operation which only produces integers.

`indirectbr` expects a block address, however these operations can't produce
that.

There were several asserts in `computeValueKnownInPredecessorsImpl` which check
that we're not looking for a block address if we're walking through something
which can never produce one.

Since it's now possible to hit these asserts, this changes them into actual
checks which return false if `Preference` is not `WantInteger`.

This adds a testcase which verifies that we don't crash anymore in these
situations.

Differential Revision: https://reviews.llvm.org/D99814