[llvm] [DAGCombiner] Honor rewrite semantics of fast-math flags in fdiv combine (PR #167595)

[Matt Arsenault via llvm-commits]

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@@ -19223,26 +19224,32 @@ SDValue DAGCombiner::visitFDIV(SDNode *N) {
                          DAG.getConstantFP(Recip, DL, VT));
   }
 
-  if (Flags.hasAllowReciprocal()) {
+  // Rewriting 1 / sqrt(Y) to rsqrt(Y) requires contract
+  if (sd_match(N, m_Flags<SDNodeFlags::AllowReciprocal |
+                          SDNodeFlags::AllowContract>())) {
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arsenm wrote:

m_Flags shouldn't allow 0 operands. It should require binding the source value like every other pattern. For the freestanding flag match, what was here before was fine 

https://github.com/llvm/llvm-project/pull/167595