[ConstantRange] Calculate precise range for multiply by -1

These are pretty common in SCEV, so make sure we get a precise
result by mapping to the sub() operation.

llvm/lib/IR/ConstantRange.cpp

@@ -1095,6 +1095,20 @@ ConstantRange::multiply(const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())
     return getEmpty();
 
+  if (const APInt *C = getSingleElement()) {
+    if (C->isOne())
+      return Other;
+    if (C->isAllOnes())
+      return ConstantRange(APInt::getZero(getBitWidth())).sub(Other);
+  }
+
+  if (const APInt *C = Other.getSingleElement()) {
+    if (C->isOne())
+      return *this;
+    if (C->isAllOnes())
+      return ConstantRange(APInt::getZero(getBitWidth())).sub(*this);
+  }
+
   // Multiplication is signedness-independent. However different ranges can be
   // obtained depending on how the input ranges are treated. These different
   // ranges are all conservatively correct, but one might be better than the

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -997,6 +997,26 @@ TEST_F(ConstantRangeTest, Multiply) {
   EXPECT_EQ(ConstantRange(APInt(8, -2)).multiply(
               ConstantRange(APInt(8, 0), APInt(8, 2))),
             ConstantRange(APInt(8, -2), APInt(8, 1)));
+
+  // Multiplication by -1 should give precise results.
+  EXPECT_EQ(ConstantRange(APInt(8, 3), APInt(8, -11))
+                .multiply(ConstantRange(APInt(8, -1))),
+            ConstantRange(APInt(8, 12), APInt(8, -2)));
+  EXPECT_EQ(ConstantRange(APInt(8, -1))
+                .multiply(ConstantRange(APInt(8, 3), APInt(8, -11))),
+            ConstantRange(APInt(8, 12), APInt(8, -2)));
+
+  TestBinaryOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.multiply(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) {
+        return N1 * N2;
+      },
+      PreferSmallest,
+      [](const ConstantRange &, const ConstantRange &) {
+        return false; // Check correctness only.
+      });
 }
 
 TEST_F(ConstantRangeTest, smul_fast) {