[CostModel] Avoid traditional ConstantExpr crashy pitfails

I'm not sure if this is a regression from D81448 + D81643,
which moved at least the code cast from elsewhere,
or somehow no one triggered that before.
But now we can reach it with a non-instruction..

It is not straight-forward to write cost-model tests for constantexprs,
`-cost-model -analyze -cost-kind=` does not appear to look at them,
or maybe i'm doing it wrong.

I've encountered that via a SimplifyCFG crash,
so reduced (currently-crashing) test is added.
There are likely other instances.

For now, simply restore previous status quo of
not crashing and returning TTI::TCC_Basic.