TypeConversion: do not materialize conversion of the type to itself

Type conversion does not necessarily affect all types, some of them may remain
untouched.  The type conversion tool from the dialect conversion framework will
unconditionally insert a temporary cast operation from the type to itself
anyway, and will try to materialize it to a real conversion operation if there
are remaining uses.  Simply use the original value instead.

PiperOrigin-RevId: 255975450