[SimpleLoopUnswitch] Forget loops before invalidating IR.

Invalidate SCEV before adjusting switch instruction, so the IR remains
in a valid state for SCEV invalidation.

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

@@ -791,15 +791,25 @@ static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
   Loop *OuterL = &L;
 
   if (DefaultExitBB) {
-    // Clear out the default destination temporarily to allow accurate
-    // predecessor lists to be examined below.
-    SI.setDefaultDest(nullptr);
     // Check the loop containing this exit.
     Loop *ExitL = LI.getLoopFor(DefaultExitBB);
     if (!ExitL || ExitL->contains(OuterL))
       OuterL = ExitL;
   }
 
+  if (SE) {
+    if (OuterL)
+      SE->forgetLoop(OuterL);
+    else
+      SE->forgetTopmostLoop(&L);
+  }
+
+  if (DefaultExitBB) {
+    // Clear out the default destination temporarily to allow accurate
+    // predecessor lists to be examined below.
+    SI.setDefaultDest(nullptr);
+  }
+
   // Store the exit cases into a separate data structure and remove them from
   // the switch.
   SmallVector<std::tuple<ConstantInt *, BasicBlock *,
@@ -822,13 +832,6 @@ static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
     SIW.removeCase(CaseI);
   }
 
-  if (SE) {
-    if (OuterL)
-      SE->forgetLoop(OuterL);
-    else
-      SE->forgetTopmostLoop(&L);
-  }
-
   // Check if after this all of the remaining cases point at the same
   // successor.
   BasicBlock *CommonSuccBB = nullptr;