IdealLoopTree::split_outer_loop leaves behind phi-nodes with only one input. When moving phis to outer loops, it needs to register the old_phis for transform, so that we can get rid of them. These phis will otherwise keep pinning other ops to a loop.

This issue was reproduced in the ZGC repo running SPECjvm98/mpegaudio compiling "spec.benchmarks._222_mpegaudio.p::g"