[llvm] [DAG] Add basic ISD::IS_FPCLASS constant/identity folds (PR #189944)

[Matt Arsenault via llvm-commits]

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@@ -16012,6 +16014,26 @@ SDValue DAGCombiner::visitAssertAlign(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitIS_FPCLASS(SDNode *N) {
+  SDValue Src = N->getOperand(0);
+  FPClassTest Mask = static_cast<FPClassTest>(N->getConstantOperandVal(1));
+  EVT VT = N->getValueType(0);
+  SDLoc DL(N);
+
+  KnownFPClass Known = DAG.computeKnownFPClass(Src, Mask);
+
+  // Clear test bits we know must be false from the source value.
+  // fp_class (nnan x), qnan|snan|other -> fp_class (nnan x), other
+  // fp_class (ninf x), ninf|pinf|other -> fp_class (ninf x), other
+  if ((Mask & Known.KnownFPClasses) != Mask)
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arsenm wrote:

Braces 

https://github.com/llvm/llvm-project/pull/189944