[llvm] [LoopPeel] LCSSA form is destroyed by LoopPeel, preserve it (PR #78696)

[Nikita Popov via llvm-commits]

nikic wrote:

@vedantparanjape-amd If the LCSSA of the parent loop is already destroyed by the time we enter LoopPeel, then I don't think trying to recover it inside LoopPeel is the right thing to do. We should fix this at the point where LCSSA is broken.

Do I understand correctly that the only relation LoopPeel has to anything here is that it contains this LCSSA assertion? It looks like adding
```
  for (Loop *L : LI)
    assert(L->isRecursivelyLCSSAForm(DT, LI));
```
to the end of unrolling triggers the assertion even without peeling.

-----

For the record, here is the test case with cleaned up block naming:
```llvm
target triple = "x86_64-unknown-linux-gnu"

define void @test() !prof !0 {
entry:
  br label %loop1

loop1:
  %phi = phi i32 [ 1, %entry ], [ 0, %loop1.latch ]
  br label %loop2

loop2:
  %phi3 = phi i64 [ 0, %loop1 ], [ %sext, %loop2.latch ]
  br label %loop3

loop3:
  %phi5 = phi i64 [ %phi3, %loop2 ], [ %sext, %loop3.latch ]
  %phi6 = phi i32 [ 1, %loop2 ], [ %add10, %loop3.latch ]
  %trunc = trunc i64 %phi5 to i32
  br i1 true, label %loop3.latch, label %exit

loop3.latch:
  %add = add i32 1, %phi
  %sext = sext i32 %add to i64
  %add10 = add i32 %phi6, 1
  %icmp = icmp ugt i32 %add10, 2
  br i1 %icmp, label %loop2.latch, label %loop3

loop2.latch:
  br i1 false, label %loop4.preheader, label %loop2

loop4.preheader:
  br label %loop4

loop4:
  br i1 false, label %loop1.latch, label %loop4

loop1.latch:
  br label %loop1

exit:
  %phi8 = phi i32 [ %trunc, %loop3 ]
  ret void
}

!0 = !{!"function_entry_count", i64 32768}
!1 = !{!"branch_weights", i32 1, i32 1}
```

https://github.com/llvm/llvm-project/pull/78696