bfd2c216ea
https://llvm.org/PR48296 shows an example where we delete all of the operands of a phi without actually deleting the phi, and that is currently considered invalid IR. The reduced test included here would crash for that reason. A suggested follow-up is to loosen the assert to allow 0-operand phis in unreachable blocks. Differential Revision: https://reviews.llvm.org/D92247