[mlir][Arith] Make previous load-bearing assert into a real error

When I landed the EmulateUnsupportedFloats, I'd negligently included
an assert that needed to run for the pass to be correct. Previous
emergency fix commits removed the assert. This commit re-adds the
"can't happen" testing as an emitOpError() and aborting the rewrite,
thus allowing it to function in no-assertions builds.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D155088