Sen. Elizabeth Warren, D-Mass., is under fire after Spirit Airlines abruptly shut down, with critics citing her claim that blocking a merger that could have saved the troubled carrier was “a Biden win for flyers.” Spirit announced early Saturday it would cease operations immediately, canceling all flights and shutting down customer service, leaving many travelers stranded. The collapse is reigniting debate over whether federal regulators got it wrong in blocking a proposed JetBlue-Spirit merger, with opponents now arguing the decision may have reduced competition and contributed to the airline’s downfall. “I’ve warned for months that a @JetBlue-@SpiritAirlines merger would have led to fewer…