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

[Matthias Springer via llvm-commits]

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@@ -290,6 +282,12 @@ const fltSemantics &APFloatBase::x87DoubleExtended() {
 }
 const fltSemantics &APFloatBase::Bogus() { return semBogus; }
 
+bool APFloatBase::isRepresentableBy(const fltSemantics &A,
+                                    const fltSemantics &B) {
+  return A.maxExponent <= B.maxExponent && A.minExponent >= B.minExponent &&
+         A.precision <= B.precision;
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matthias-springer wrote:

I think this should take into account `hasZero` and `hasSignedRepr`. Or the documentation should make clear that it doesn't. 

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