Transforms: Fix bootstrap after r266565

Apparently there isn't test coverage for all of these.  I'd appreciate
if someone with could reproduce and send me something to reduce, but for
now I've just looked for users of RemapInstruction and MapValue and
ensured they don't accidentally insert nullptr.  Here is one of the
bootstraps that caught:

  http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/11494

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266567 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopRotation.cpp

@@ -70,7 +70,7 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
     if (OrigHeaderVal->use_empty())
       continue;
 
-    Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];
+    Value *OrigPreHeaderVal = ValueMap.lookup(OrigHeaderVal);
 
     // The value now exits in two versions: the initial value in the preheader
     // and the loop "next" value in the original header.

lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -94,7 +94,7 @@ static void ConnectProlog(Loop *L, Value *BECount, unsigned Count,
       Value *V = PN->getIncomingValueForBlock(Latch);
       if (Instruction *I = dyn_cast<Instruction>(V)) {
         if (L->contains(I)) {
-          V = VMap[I];
+          V = VMap.lookup(I);
         }
       }
       // Adding a value to the new PHI node from the last prolog block
@@ -199,7 +199,7 @@ static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
     Instruction *I = dyn_cast<Instruction>(V);
     if (I && L->contains(I))
       // If value comes from an instruction in the loop add VMap value.
-      V = VMap[I];
+      V = VMap.lookup(I);
     // For the instruction out of the loop, constant or undefined value
     // insert value itself.
     EpilogPN->addIncoming(V, EpilogLatch);
@@ -353,8 +353,8 @@ static void CloneLoopBlocks(Loop *L, Value *NewIter,
       idx = NewPHI->getBasicBlockIndex(Latch);
       Value *InVal = NewPHI->getIncomingValue(idx);
       NewPHI->setIncomingBlock(idx, NewLatch);
-      if (VMap[InVal])
-        NewPHI->setIncomingValue(idx, VMap[InVal]);
+      if (Value *V = VMap.lookup(InVal))
+        NewPHI->setIncomingValue(idx, V);
     }
   }
   if (NewLoop) {