[IR][LoopRotate] avoid leaving phi with no operands (PR48296)

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