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

Both LLVM and SPIR-V have some form of "is this float a NaN/Inf"
operation (though LLVM's uses the rather opaque "is.fpclass"
intrinsic), which is not exposed in MLIR.

This has lead to awkward workarounds in -arith-expands-ops where a NaN
test was performed by comparing an operation to itself. This commit
resolves that issue.

Reviewed By: dcaballe, kuhar

Differential Revision: https://reviews.llvm.org/D156169