x86-64: Generate push2/pop2 only if the incoming stack is 16-byte aligned

Since push2/pop2 requires 16-byte stack alignment, don't generate them
if the incoming stack isn't 16-byte aligned.

gcc/

	PR target/113912
	* config/i386/i386.cc (ix86_can_use_push2pop2): New.
	(ix86_pro_and_epilogue_can_use_push2pop2): Use it.
	(ix86_emit_save_regs): Don't generate push2 if
	ix86_can_use_push2pop2 return false.
	(ix86_expand_epilogue): Don't generate pop2 if
	ix86_can_use_push2pop2 return false.

gcc/testsuite/

	PR target/113912
	* gcc.target/i386/apx-push2pop2-2.c: New test.