[IR][LoopRotate] remove assertion that phi must have at least one operand

This was suggested in D92247 - I initially committed an alternate
fix ( bfd2c216 ) to avoid the crash/assert shown in
https://llvm.org/PR48296 ,
but that was reverted because it caused msan failures on other
tests. We can try to revive that patch using the test included
here, but I do not have an immediate plan to isolate that problem.