[APInt] APInt::clearBitsSlowCase - fix cut+paste typo when merging lo/himasks (#141108)

Fixes #141098

Author: RKSimon

llvm/lib/Support/APInt.cpp

@@ -349,9 +349,9 @@ void APInt::clearBitsSlowCase(unsigned LoBit, unsigned HiBit) {
     // Create a high mask with ones above HiBit.
     uint64_t HiMask = ~(WORDTYPE_MAX >> (APINT_BITS_PER_WORD - HiShiftAmt));
     // If LoWord and HiWord are equal, then we combine the masks. Otherwise,
-    // set the bits in HiWord.
+    // clear the bits in HiWord.
     if (HiWord == LoWord)
-      LoMask &= HiMask;
+      LoMask |= HiMask;
     else
       U.pVal[HiWord] &= HiMask;
   }

llvm/unittests/ADT/APIntTest.cpp

@@ -2556,6 +2556,15 @@ TEST(APIntTest, clearBits) {
   EXPECT_EQ(6u, i256.countl_one());
   EXPECT_EQ(16u, i256.popcount());
 
+  APInt i299 = APInt::getAllOnes(299);
+  i299.clearBits(240, 250);
+  EXPECT_EQ(240u, i299.countr_one());
+  EXPECT_EQ(0u, i299.countr_zero());
+  EXPECT_EQ(299u, i299.getActiveBits());
+  EXPECT_EQ(0u, i299.countl_zero());
+  EXPECT_EQ(49u, i299.countl_one());
+  EXPECT_EQ(289u, i299.popcount());
+
   APInt i311 = APInt::getAllOnes(311);
   i311.clearBits(33, 99);
   EXPECT_EQ(33u, i311.countr_one());