[llvm] [DAGCombine] Add DAG optimisation for BF16_TO_FP (PR #69426)

[Craig Topper via llvm-commits]

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@@ -26219,6 +26223,21 @@ SDValue DAGCombiner::visitFP_TO_BF16(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitBF16_TO_FP(SDNode *N) {
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topperc wrote:

You need to use `APInt DemandedBits = APInt::getLoBitsSet(N0.getScalarValueSizeInBits(), 16);` so that only the lower 16 bits are demanded.

https://github.com/llvm/llvm-project/pull/69426