[MLIR][EmitC] Don't translate expressions inline if user is `emitc.subscript` (#91087)

This change updates the logic that determines whether an `emitc.expression`
result is translated into a dedicated variable assignment. Due to how
the translation of `emitc.subscript` currently works, a previously
inline-able `emitc.expression` would produce incorrect C++ if its single user 
was a `emitc.subscript` operation.