Fix bitcasting E8M0 APFloat to APInt (#113298)

Fixes a bug in APFloat handling of E8M0 type (zero mantissa).

Related PRs:
- #107127
- #111028

llvm/lib/Support/APFloat.cpp

@@ -408,7 +408,8 @@ exponentNaN(const fltSemantics &semantics) {
   if (semantics.nonFiniteBehavior == fltNonfiniteBehavior::NanOnly) {
     if (semantics.nanEncoding == fltNanEncoding::NegativeZero)
       return exponentZero(semantics);
-    return semantics.maxExponent;
+    if (semantics.hasSignedRepr)
+      return semantics.maxExponent;
   }
   return semantics.maxExponent + 1;
 }
@@ -3663,7 +3664,7 @@ APInt IEEEFloat::convertIEEEFloatToAPInt() const {
   std::array<uint64_t, (S.sizeInBits + 63) / 64> words;
   auto words_iter =
       std::copy_n(mysignificand.begin(), mysignificand.size(), words.begin());
-  if constexpr (significand_mask != 0) {
+  if constexpr (significand_mask != 0 || trailing_significand_bits == 0) {
     // Clear the integer bit.
     words[mysignificand.size() - 1] &= significand_mask;
   }

llvm/unittests/ADT/APFloatTest.cpp

@@ -5985,6 +5985,9 @@ TEST(APFloatTest, Float8E8M0FNUExhaustive) {
     APFloat test(APFloat::Float8E8M0FNU(), APInt(8, i));
     SCOPED_TRACE("i=" + std::to_string(i));
 
+    // bitcastToAPInt
+    EXPECT_EQ(i, test.bitcastToAPInt());
+
     // isLargest
     if (i == 254) {
       EXPECT_TRUE(test.isLargest());