[CVP] Don't use undef range for LHS of div/rem transforms

Using it for RHS is fine, as undef is UB in that case.

Author: nikic

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -823,8 +823,11 @@ static bool processUDivOrURem(BinaryOperator *Instr, LazyValueInfo *LVI) {
   if (Instr->getType()->isVectorTy())
     return false;
 
-  ConstantRange XCR = LVI->getConstantRangeAtUse(Instr->getOperandUse(0));
-  ConstantRange YCR = LVI->getConstantRangeAtUse(Instr->getOperandUse(1));
+  ConstantRange XCR = LVI->getConstantRangeAtUse(Instr->getOperandUse(0),
+                                                 /*UndefAllowed*/ false);
+  // Allow undef for RHS, as we can assume it is division by zero UB.
+  ConstantRange YCR = LVI->getConstantRangeAtUse(Instr->getOperandUse(1),
+                                                 /*UndefAllowed*/ true);
   if (expandUDivOrURem(Instr, XCR, YCR))
     return true;
 
@@ -951,8 +954,11 @@ static bool processSDivOrSRem(BinaryOperator *Instr, LazyValueInfo *LVI) {
   if (Instr->getType()->isVectorTy())
     return false;
 
-  ConstantRange LCR = LVI->getConstantRangeAtUse(Instr->getOperandUse(0));
-  ConstantRange RCR = LVI->getConstantRangeAtUse(Instr->getOperandUse(1));
+  ConstantRange LCR =
+      LVI->getConstantRangeAtUse(Instr->getOperandUse(0), /*AllowUndef*/ false);
+  // Allow undef for RHS, as we can assume it is division by zero UB.
+  ConstantRange RCR =
+      LVI->getConstantRangeAtUse(Instr->getOperandUse(1), /*AlloweUndef*/ true);
   if (Instr->getOpcode() == Instruction::SDiv)
     if (processSDiv(Instr, LCR, RCR, LVI))
       return true;

llvm/test/Transforms/CorrelatedValuePropagation/urem.ll

@@ -394,7 +394,6 @@ else:
   ret void
 }
 
-; FIXME: This is a miscompile.
 define i8 @urem_undef_range_op1(i8 %x) {
 ; CHECK-LABEL: @urem_undef_range_op1(
 ; CHECK-NEXT:  entry:
@@ -408,7 +407,8 @@ define i8 @urem_undef_range_op1(i8 %x) {
 ; CHECK-NEXT:    br label [[JOIN]]
 ; CHECK:       join:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi i8 [ 1, [[CASE1]] ], [ 2, [[CASE2]] ], [ undef, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    ret i8 [[PHI]]
+; CHECK-NEXT:    [[RES:%.*]] = urem i8 [[PHI]], 3
+; CHECK-NEXT:    ret i8 [[RES]]
 ;
 entry:
   switch i8 %x, label %join [