MLIR][STD] Fold trunci (sexti).

This patch folds the following pattern:

```
%arg0 = ...
%0 = sexti %arg0 : i1 to i8
%1 = trunci %0 : i8 to i1
```

into just `%arg0`.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D99464