[mlir][tosa] Fix argmax folder when output type is i64 (#163583)
Previously the following IR:
```
tosa.argmax %arg0 {axis = 0 : i32} : (tensor<1xi8>) -> tensor<i64>
```
Would result in a crash with the assertion:
```
expected dense element bit width 64 to match data size 32 for type i64
```
This commit ensures that zero is constructed with the correct bitwidth
while folding, therefore fixing the crash.