[llvm] [KnownFPClass] Fix input/output subnormal handling for `propagateXorSign` (PR #219837)

[Matt Arsenault via llvm-commits]

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@@ -150,6 +150,18 @@ struct KnownFPClass {
     return isKnownNever(fcPositive) && isKnownNeverLogicalNegZero(Mode);
   }
 
+  /// Return true if this cannot have a non-NaN positive value when interpreted
+  /// as an input under \p Mode.
+  bool cannotHavePositiveValue(DenormalMode Mode) const {
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arsenm wrote:

Isn't this the intent of signBitMustBeZero / signBitMustBeOne?

We already have cannotBeOrderedLessThanZero / cannotBeOrderedGreaterThanZero, then need the sign bit specific function for the 0 special case 

https://github.com/llvm/llvm-project/pull/219837