[X86/Atomics] Correct a few transforms for new atomic lowering

This is a partial fix for the issues described in commit message of 027aa27d (the revert of G24609).  Unfortunately, I can't provide test coverage for it on it's own as the only (known) wrong example is still wrong, but due to a separate issue.

These fixes are cases where when performing unrelated DAG combines, we were dropping the atomicity flags entirely.