[SimplifyCFG] FoldBranchToCommonDest: gracefully handle unreachable code ()

We might be dealing with an unreachable code,
so the bonus instruction we clone might be self-referencing.

There is a sanity check that all uses of bonus instructions
that are not in the original block with said bonus instructions
are PHI nodes, and that is obviously not the case
for self-referencing instructions..

So if we find such an use, just rewrite it.

Thanks to Mikael Holmén for the reproducer!

Fixes https://bugs.llvm.org/show_bug.cgi?id=48450#c8

Author: LebedevRI

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2986,6 +2986,13 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
                      "Non-external users are never PHI instructions.");
               return false;
             }
+            if (User->getParent() == PredBlock) {
+              // The "exteral" use is in the block into which we just cloned the
+              // bonus instruction. This means two things: 1. we are in an
+              // unreachable block 2. the instruction is self-referencing.
+              // So let's just rewrite it...
+              return true;
+            }
             (void)BI;
             assert(isa<PHINode>(User) && "All external users must be PHI's.");
             auto *PN = cast<PHINode>(User);

llvm/test/Transforms/SimplifyCFG/fold-branch-to-common-dest.ll

@@ -836,3 +836,57 @@ for.bodythread-pre-split.loopback:
 if.end.loopexit:
   ret void
 }
+
+@f.b = external global i16, align 1
+define void @pr48450_3() {
+; CHECK-LABEL: @pr48450_3(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND1:%.*]]
+; CHECK:       for.cond1:
+; CHECK-NEXT:    [[V:%.*]] = load i16, i16* @f.b, align 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i16 [[V]], 1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[CMP]])
+; CHECK-NEXT:    br label [[FOR_COND1]]
+;
+entry:
+  br label %for.cond1
+
+for.cond1:
+  %v = load i16, i16* @f.b, align 1
+  %cmp = icmp slt i16 %v, 1
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:
+  br label %for.cond1
+
+for.end:
+  %tobool = icmp ne i16 %v, 0
+  br i1 %tobool, label %if.then, label %if.end
+
+if.then:
+  unreachable
+
+if.end:
+  br label %for.cond2
+
+for.cond2:
+  %c.0 = phi i16 [ undef, %if.end ], [ %inc, %if.end7 ]
+  %cmp3 = icmp slt i16 %c.0, 1
+  br i1 %cmp3, label %for.body4, label %for.cond.cleanup
+
+for.cond.cleanup:
+  br label %cleanup
+
+for.body4:
+  br i1 undef, label %if.then6, label %if.end7
+
+if.then6:
+  br label %cleanup
+
+if.end7:
+  %inc = add nsw i16 %c.0, 1
+  br label %for.cond2
+
+cleanup:
+  unreachable
+}