[llvm-dev] When can the dominator tree not contain a node for a basic block?

[Philip Reames via llvm-dev]

When looking into https://llvm.org/bugs/show_bug.cgi?id=24866, I 
discovered that the root cause of the crash is that I was expecting 
every basic block to have a corresponding Node in the dominator tree.  
Apparently, the "while.end" basic block in the example does not have a 
Node in the Dominator Tree.  Can anyone tell me if this is expected?   
If so, under what circumstances?

Interestingly, the crash in question appears to be fairly recently 
introduced.  A checkout from week before last didn't crash, whereas a 
checkout from this morning does.  I have yet tried to isolate the change 
in question; before doing so, I wanted to determine if this was a newly 
introduced problem (i.e. every BB should have a Node), or merely a newly 
exposed problem (i.e. not every BB does).

Philip

For reference, here's the reproduction command:
opt -S <input> -value-tracking-dom-conditions -licm -load-combine

And here's the IR:
; ModuleID = '../bugpoint-reduced-simplified.bc'
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc18.0.0"

%struct.c_derived_tbl.2.5.8.11.14.17.23.38.59.80.92.98.104.107.155.183 = 
type { [256 x i32], [256 x i8] }

; Function Attrs: nounwind uwtable
define void 
@encode_one_blockX2(%struct.c_derived_tbl.2.5.8.11.14.17.23.38.59.80.92.98.104.107.155.183* 
nocapture readonly %actbl) #0 {
entry:
   br i1 undef, label %L_KLOOP_01, label %L_KLOOP.preheader

L_KLOOP_01:                                       ; preds = %while.end, 
%entry
   br label %L_KLOOP.preheader

L_KLOOP_08:                                       ; preds = %while.end
   br label %L_KLOOP.preheader

L_KLOOP.preheader:                                ; preds = %L_KLOOP_08, 
%L_KLOOP_01, %entry
   %r.2.ph = phi i32 [ undef, %L_KLOOP_08 ], [ 0, %entry ], [ undef, 
%L_KLOOP_01 ]
   br label %L_KLOOP

L_KLOOP:                                          ; preds = %while.end, 
%L_KLOOP.preheader
   %r.2 = phi i32 [ 0, %while.end ], [ %r.2.ph, %L_KLOOP.preheader ]
   br i1 undef, label %while.body, label %while.end

while.body:                                       ; preds = %while.body, 
%L_KLOOP
   br label %while.body

while.end:                                        ; preds = %L_KLOOP
   %shl105 = shl i32 %r.2, 4
   %add106 = add nsw i32 %shl105, undef ;; <-- THIS is the context 
instruction.  V is undef
   %idxprom107 = sext i32 %add106 to i64
   %arrayidx108 = getelementptr inbounds 
%struct.c_derived_tbl.2.5.8.11.14.17.23.38.59.80.92.98.104.107.155.183, 
%struct.c_derived_tbl.2.5.8.11.14.17.23.38.59.80.92.98.104.107.155.183* 
%actbl, i64 0, i32 0, i64 %idxprom107
   %0 = load i32, i32* %arrayidx108, align 4, !tbaa !2
   %arrayidx110 = getelementptr inbounds 
%struct.c_derived_tbl.2.5.8.11.14.17.23.38.59.80.92.98.104.107.155.183, 
%struct.c_derived_tbl.2.5.8.11.14.17.23.38.59.80.92.98.104.107.155.183* 
%actbl, i64 0, i32 1, i64 %idxprom107
   %1 = load i8, i8* %arrayidx110, align 1, !tbaa !6
   indirectbr i8* undef, [label %L_KLOOP_DONE, label %L_KLOOP_01, label 
%L_KLOOP_08, label %L_KLOOP]

L_KLOOP_DONE:                                     ; preds = %while.end
   ret void
}

attributes #0 = { nounwind uwtable "disable-tail-calls"="false" 
"less-precise-fpmad"="false" "no-frame-pointer-elim"="false" 
"no-infs-fp-math"="false" "no-nans-fp-math"="false" 
"stack-protector-buffer-size"="8" "target-cpu"="x86-64" 
"target-features"="+sse,+sse2" "unsafe-fp-math"="false" 
"use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"clang version 3.8.0 (http://llvm.org/git/clang.git 
37c860b7d0d43dde1edcd324e00a10a62b2c5776) (http://llvm.org/git/llvm.git 
fb73c0262af4183555253a5c8c13025a6e7630bc)"}
!2 = !{!3, !3, i64 0}
!3 = !{!"int", !4, i64 0}
!4 = !{!"omnipotent char", !5, i64 0}
!5 = !{!"Simple C/C++ TBAA"}
!6 = !{!4, !4, i64 0}