[llvm] [X86][BF16] Do not combine FP_TRUNC + FP_EXTEND if they come from user (PR #91420)

[Phoebe Wang via llvm-commits]

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@@ -56732,6 +56732,7 @@ static SDValue combineFP16_TO_FP(SDNode *N, SelectionDAG &DAG,
 }
 
 static SDValue combineFP_EXTEND(SDNode *N, SelectionDAG &DAG,
+                                TargetLowering::DAGCombinerInfo &DCI,
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phoebewang wrote:

Legalizer usually only focuses on a single node, while combiner works for multi nodes.

I think we can do it to `half` too. Let's start from BF16 and see if there's any other objections.

https://github.com/llvm/llvm-project/pull/91420