Backport of part of D71181.

This backports *just* the bug fix portion of

llvm@a7d992c#diff-1f9547b463bf33c1dd86f485d0fa410eR1465

Author: pnkfelix

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -543,10 +543,16 @@ static Value *ThreadCmpOverPHI(CmpInst::Predicate Pred, Value *LHS, Value *RHS,
 
   // Evaluate the BinOp on the incoming phi values.
   Value *CommonValue = nullptr;
-  for (Value *Incoming : PI->incoming_values()) {
+  for (unsigned u = 0, e = PI->getNumIncomingValues(); u < e; ++u) {
+    Value *Incoming = PI->getIncomingValue(u);
+    Instruction *InTI = PI->getIncomingBlock(u)->getTerminator();
     // If the incoming value is the phi node itself, it can safely be skipped.
     if (Incoming == PI) continue;
-    Value *V = SimplifyCmpInst(Pred, Incoming, RHS, Q, MaxRecurse);
+    // Change the context instruction to the "edge" that flows into the phi.
+    // This is important because that is where incoming is actually "evaluated"
+    // even though it is used later somewhere else.
+    Value *V = SimplifyCmpInst(Pred, Incoming, RHS, Q.getWithInstruction(InTI),
+                               MaxRecurse);
     // If the operation failed to simplify, or simplified to a different value
     // to previously, then give up.
     if (!V || (CommonValue && V != CommonValue))

llvm/lib/Analysis/ValueTracking.cpp

@@ -1346,6 +1346,8 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
       for (unsigned i = 0; i != 2; ++i) {
         Value *L = P->getIncomingValue(i);
         Value *R = P->getIncomingValue(!i);
+        Instruction *RInst = P->getIncomingBlock(!i)->getTerminator();
+        Instruction *LInst = P->getIncomingBlock(i)->getTerminator();
         Operator *LU = dyn_cast<Operator>(L);
         if (!LU)
           continue;
@@ -1367,13 +1369,22 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
             L = LL;
           else
             break;
+
+          // Change the context instruction to the "edge" that flows into the
+          // phi. This is important because that is where the value is actually
+          // "evaluated" even though it is used later somewhere else. (see also
+          // D69571).
+          Query RecQ = Q;
+
           // Ok, we have a PHI of the form L op= R. Check for low
           // zero bits.
-          computeKnownBits(R, Known2, Depth + 1, Q);
+          RecQ.CxtI = RInst;
+          computeKnownBits(R, Known2, Depth + 1, RecQ);
 
           // We need to take the minimum number of known bits
           KnownBits Known3(Known);
-          computeKnownBits(L, Known3, Depth + 1, Q);
+          RecQ.CxtI = LInst;
+          computeKnownBits(L, Known3, Depth + 1, RecQ);
 
           Known.Zero.setLowBits(std::min(Known2.countMinTrailingZeros(),
                                          Known3.countMinTrailingZeros()));
@@ -1429,14 +1440,22 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
 
       Known.Zero.setAllBits();
       Known.One.setAllBits();
-      for (Value *IncValue : P->incoming_values()) {
+      for (unsigned u = 0, e = P->getNumIncomingValues(); u < e; ++u) {
+        Value *IncValue = P->getIncomingValue(u);
         // Skip direct self references.
         if (IncValue == P) continue;
 
+        // Change the context instruction to the "edge" that flows into the
+        // phi. This is important because that is where the value is actually
+        // "evaluated" even though it is used later somewhere else. (see also
+        // D69571).
+        Query RecQ = Q;
+        RecQ.CxtI = P->getIncomingBlock(u)->getTerminator();
+
         Known2 = KnownBits(BitWidth);
         // Recurse, but cap the recursion to one level, because we don't
         // want to waste time spinning around in loops.
-        computeKnownBits(IncValue, Known2, MaxDepth - 1, Q);
+        computeKnownBits(IncValue, Known2, MaxDepth - 1, RecQ);
         Known.Zero &= Known2.Zero;
         Known.One &= Known2.One;
         // If all bits have been ruled out, there's no need to check