[Polly] Ensure consistent Scop::InstStmtMap. NFC.

InstStmtMap became inconsistent with ScopStmt::getInstructions() after
the statement's instructions is modified, e.g. by being considered
unused by the Simplify pass or being moved by ForwardOpTree.

Change ScopStmt::setInstructions() to also update its parent's
InstStmtMap. Also add assertions checking the consistency.