[GlobalISel] Add IRTranslator support for G_ISNAN

Translate the `@llvm.isnan` intrinsic to G_ISNAN when we see it.

This is pretty much the same as the associated SelectionDAGBuilder code. Main
difference is that we don't expand it here. It makes more sense to do that
during legalization in GlobalISel. GlobalISel will just legalize the generated
illegal types.

Differential Revision: https://reviews.llvm.org/D108226