[RemoveDIs] Insert PHIs before debug records in hardware loops (#85288)

Fixes: https://github.com/llvm/llvm-project/issues/85254

Hardware loops inserts PHIs at the position `getFirstNonPhi()`, which is
incorrect - instead, `getFirstNonPhiIt()` is required to not insert the
PHI after any debug records that immediately follow the last existing
PHI.