Revert "[mlir][Arith] Add arith.is_nan and arith.is_inf predicates"

This reverts commit 7c349c36.

Per discussion at
https://reviews.llvm.org/rG7c349c369847dc2f1736efb9c90d03521cd44a90
and elsewhere, the lowering to LLVM defined here isn't what it should
be and the fastmath flag usage isn't correct, so `arith.is_nan` and
`arith.is_inf` cannot exist in their current form.

It's unclear if those operations should be introduced in the future,
since they make the dialect more complex and don't add any expressive
power. Further discussion may be moved to an RFC (or I'll drop this
patch).

Differential Revision: https://reviews.llvm.org/D157543