[APInt] Fix getAllOnes() with zero width

This makes sure that APInt::getAllOnes() keeps working after
the APInt constructor assertions are enabled.

I'm relaxing the requirement for the signed case to either
an all zeros or all ones integer.

Author: nikic

llvm/include/llvm/ADT/APInt.h

@@ -112,14 +112,21 @@ class [[nodiscard]] APInt {
         bool implicitTrunc = true)
       : BitWidth(numBits) {
     if (!implicitTrunc) {
-      if (BitWidth == 0) {
-        assert(val == 0 && "Value must be zero for 0-bit APInt");
-      } else if (isSigned) {
-        assert(llvm::isIntN(BitWidth, val) &&
-               "Value is not an N-bit signed value");
+      if (isSigned) {
+        if (BitWidth == 0) {
+          assert((val == 0 || val == uint64_t(-1)) &&
+                 "Value must be 0 or -1 for signed 0-bit APInt");
+        } else {
+          assert(llvm::isIntN(BitWidth, val) &&
+                 "Value is not an N-bit signed value");
+        }
       } else {
-        assert(llvm::isUIntN(BitWidth, val) &&
-               "Value is not an N-bit unsigned value");
+        if (BitWidth == 0) {
+          assert(val == 0 && "Value must be zero for unsigned 0-bit APInt");
+        } else {
+          assert(llvm::isUIntN(BitWidth, val) &&
+                 "Value is not an N-bit unsigned value");
+        }
       }
     }
     if (isSingleWord()) {

llvm/unittests/ADT/APIntTest.cpp

@@ -3423,6 +3423,7 @@ TEST(APIntTest, ZeroWidth) {
   EXPECT_EQ(0U, ZW.getBitWidth());
   EXPECT_EQ(0U, APInt(0, ArrayRef<uint64_t>({0, 1, 2})).getBitWidth());
   EXPECT_EQ(0U, APInt(0, "0", 10).getBitWidth());
+  EXPECT_EQ(0U, APInt::getAllOnes(0).getBitWidth());
 
   // Default constructor is single bit wide.
   EXPECT_EQ(1U, APInt().getBitWidth());