[mlir][Arith] Specify evaluation order of `getExpr` (#87859)

The C++ standard does not specify an evaluation order for addition/...
operands. E.g., in `a() + b()`, the compiler is free to evaluate `a` or
`b` first.

This lead to different `mlir-opt` outputs in #85895. (FileCheck passed
when compiled with LLVM but failed when compiled with gcc.)