[mlir][spirv] Add IsInf/IsNan expansion for WebGPU (#86903)

These non-finite math ops are supported by SPIR-V but not by WGSL.
Assume finite floating point values and expand these ops into `false`.

Previously, this worked by adding fast math flags during conversion from
arith to spirv, but this got removed in
https://github.com/llvm/llvm-project/pull/86578.

Also do some misc cleanups in the surrounding code.