[flang] Do not create arith.extui with same from/to type

In some case the lowering of `ichar` is generating an `arith.extui` operation
with the same from/to type. This operation do not accept from/to types to be
the same. If the from/to types are identical, we do not generate the extra
operation.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D124107