[llvm] [APFloat][NFC]extract `fltSemantics::isRepresentableBy` to header (PR #122636)

[Durgadoss R via llvm-commits]

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@@ -281,6 +281,11 @@ struct APFloatBase {
   /// anything real.
   static const fltSemantics &Bogus() LLVM_READNONE;
 
+  // Returns true if any number described by this semantics can be precisely
+  // represented by the specified semantics. Does not take into account
+  // the value of fltNonfiniteBehavior.
+  static bool isRepresentableBy(const fltSemantics &A, const fltSemantics &B);
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durga4github wrote:

If we are changing the semantics of this, can we do that as a separate PR?

https://github.com/llvm/llvm-project/pull/122636