Bring back APInt self-move assignment check for MSVC only

[rnk]

Summary

It was removed about a year ago in r300477. Bring it back, along with
its unittest.

[craig.topper]

LGTM

llvm/trunk/include/llvm/ADT/APInt.h

APInt &operator=(const APInt &RHS) {
     }
 
     AssignSlowCase(RHS);
     return *this;
   }
 
   /// @brief Move assignment operator.
   APInt &operator=(APInt &&that) {
+#ifdef _MSC_VER
+    // The MSVC std::shuffle implementation still does self-assignment.
+    if (this == &that)
+      return *this;
+#endif
     assert(this != &that && "Self-move not supported");
     if (!isSingleWord())
       delete[] U.pVal;
 
     // Use memcpy so that type based alias analysis sees both VAL and pVal
     // as modified.
     memcpy(&U, &that.U, sizeof(U));
 

llvm/trunk/unittests/ADT/APIntTest.cpp

for (int N : { 1, 2, 3, 4, 7, 8, 16, 32, 64, 127, 128, 129, 256 }) {
     }
     for (int I = 1; I < N; ++I) {
       APInt MaskVal = APInt::getHighBitsSet(N, I);
       EXPECT_TRUE(MaskVal.isShiftedMask());
     }
   }
 }
 
+// Test that self-move works, but only when we're using MSVC.
+#if defined(_MSC_VER)
+#if defined(__clang__)
+// Disable the pragma warning from versions of Clang without -Wself-move
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunknown-pragmas"
+// Disable the warning that triggers on exactly what is being tested.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wself-move"
+#endif
+TEST(APIntTest, SelfMoveAssignment) {
+  APInt X(32, 0xdeadbeef);
+  X = std::move(X);
+  EXPECT_EQ(32u, X.getBitWidth());
+  EXPECT_EQ(0xdeadbeefULL, X.getLimitedValue());
+
+  uint64_t Bits[] = {0xdeadbeefdeadbeefULL, 0xdeadbeefdeadbeefULL};
+  APInt Y(128, Bits);
+  Y = std::move(Y);
+  EXPECT_EQ(128u, Y.getBitWidth());
+  EXPECT_EQ(~0ULL, Y.getLimitedValue());
+  const uint64_t *Raw = Y.getRawData();
+  EXPECT_EQ(2u, Y.getNumWords());
+  EXPECT_EQ(0xdeadbeefdeadbeefULL, Raw[0]);
+  EXPECT_EQ(0xdeadbeefdeadbeefULL, Raw[1]);
+}
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#pragma clang diagnostic pop
+#endif
+#endif // _MSC_VER
+
 TEST(APIntTest, reverseBits) {
   EXPECT_EQ(1, APInt(1, 1).reverseBits());
   EXPECT_EQ(0, APInt(1, 0).reverseBits());
 
   EXPECT_EQ(3, APInt(2, 3).reverseBits());
   EXPECT_EQ(3, APInt(2, 3).reverseBits());
 
   EXPECT_EQ(0xb, APInt(4, 0xd).reverseBits());