[llvm] [InstCombine] Make `(binop ({s|u}itofp),({s|u}itofp))` transform more flexible to mismatched signs (PR #84389)

[Nikita Popov via llvm-commits]

================
@@ -1449,14 +1435,19 @@ Instruction *InstCombinerImpl::foldFBinOpOfIntCasts(BinaryOperator &BO) {
   };
 
   auto IsNonNeg = [&](unsigned OpNo) -> bool {
-    if (OpsKnown[OpNo].hasKnownBits() &&
-        OpsKnown[OpNo].getKnownBits(SQ).isNonNegative())
-      return true;
-    return isKnownNonNegative(IntOps[OpNo], SQ);
+    // NB: This matches the impl in ValueTracking, we just try to use cached
+    // knownbits here. If we ever start supporting WithCache for
+    // `isKnownNonNegative`, change this to an explicitly call.
----------------
nikic wrote:

```suggestion
    // `isKnownNonNegative`, change this to an explicit call.
```

https://github.com/llvm/llvm-project/pull/84389