[MLIR][EmitC] arith-to-emitc: Fix lowering of fptoui (#118504)
`arith.fptoui %arg0 : f32 to i16` was lowered to
```
%0 = emitc.cast %arg0 : f32 to ui32
emitc.cast %0 : ui32 to i16
```
and is now lowered to
```
%0 = emitc.cast %arg0 : f32 to ui16
emitc.cast %0 : ui16 to i16
```