[SimplifyCFG] Fix nasty RAUW bug from r277325

Using RAUW was wrong here; if we have a switch transform such as:
  18 -> 6 then
  6 -> 0

If we use RAUW, while performing the second transform the  *transformed* 6
from the first will be also replaced, so we end up with:
  18 -> 0
  6 -> 0

Found by clang stage2 bootstrap; testcase added.

llvm-svn: 277332

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index f4d7f5e..8e821d9 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5132,11 +5132,12 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
                                Builder.CreateShl(Sub, Ty->getBitWidth() - Shift));
   SI->replaceUsesOfWith(SI->getCondition(), Rot);
 
-  for (auto &C : SI->cases()) {
+  for (SwitchInst::CaseIt C = SI->case_begin(), E = SI->case_end(); C != E;
+       ++C) {
     auto *Orig = C.getCaseValue();
     auto Sub = Orig->getValue() - APInt(Ty->getBitWidth(), Base);
-    SI->replaceUsesOfWith(Orig,
-                          ConstantInt::get(Ty, Sub.lshr(ShiftC->getValue())));
+    C.setValue(
+        cast<ConstantInt>(ConstantInt::get(Ty, Sub.lshr(ShiftC->getValue()))));
   }
   return true;
 }