Loop strip mining construction is a 2 step process: initially, the outer loop is only built as a skeleton without Phis. After loop opts, Phis are added to the outer loop. The motivation for this is to keep the outer loop as simple as possible during loop opts so loop opts can mostly ignore it's there. The drawback is a somewhat complicated second step in some corner cases.

It's possible that fully constructing the outer loop wouldn't affect loop opts that much and would be a better trade off.

See discussion:
https://github.com/openjdk/jdk/pull/25717#issuecomment-2960287377
https://github.com/openjdk/jdk/pull/25717#issuecomment-2966381509
https://github.com/openjdk/jdk/pull/25717#issuecomment-2967236228
https://github.com/openjdk/jdk/pull/25717#issuecomment-2967280652
https://github.com/openjdk/jdk/pull/25717#issuecomment-2969458368
https://github.com/openjdk/jdk/pull/25717#pullrequestreview-2934351620