[mlir][spirv] Use UnrealizedConversionCast in ArithmeticToSPIRV

This avoids pulling in function converion patterns, which is not
part of what we want to test in ArithmeticToSPIRV. It also allows
using ConvertArithmeticToSPIRVPass as a standalone step.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D127573