[llvm] [KnownFPClass] Fix canonicalize incorrectly dropping fcNegZero under positive-zero denormal mode (PR #202268)

[Matt Arsenault via llvm-commits]

================
@@ -228,8 +228,15 @@ KnownFPClass KnownFPClass::canonicalize(const KnownFPClass &KnownSrc,
 
   if (DenormMode.Input == DenormalMode::PositiveZero ||
       (DenormMode.Output == DenormalMode::PositiveZero &&
-       DenormMode.Input == DenormalMode::IEEE))
-    Known.knownNot(fcNegZero);
+       DenormMode.Input == DenormalMode::IEEE)) {
+    // -0.0 is not a subnormal, not governed by DenormalMode and canonicalize
+    // conserves its sign (canonicalize(-0.0) == -0.0).
----------------
arsenm wrote:

```suggestion
    // -0.0 is not a subnormal and should not be flushed.
```

https://github.com/llvm/llvm-project/pull/202268