[llvm] ValueTracking: Handle tracking nan through powi (PR #179311)

[Matt Arsenault via llvm-commits]

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@@ -675,9 +675,21 @@ KnownFPClass KnownFPClass::ldexp(const KnownFPClass &KnownSrc,
   return Known;
 }
 
+// TODO: Detect no-infinity cases
 KnownFPClass KnownFPClass::powi(const KnownFPClass &KnownSrc,
                                 const KnownBits &ExponentKnownBits) {
   KnownFPClass Known;
+  Known.propagateNaN(KnownSrc);
+
+  if (ExponentKnownBits.isZero()) {
+    // powi(QNaN, 0) returns 1.0, and powi(SNaN, 0) may non-deterministically
+    // return 1.0 or a NaN.
+    FPClassTest PossibleVals =
+        KnownSrc.isKnownNever(fcSNan) ? fcPosNormal | fcNan : fcPosNormal;
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arsenm wrote:

This was right the first time 

https://github.com/llvm/llvm-project/pull/179311