[llvm] [LoongArch] Fix broadcast load with extension. (PR #155960)

[via llvm-commits]

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@@ -2471,8 +2471,10 @@ static SDValue lowerBUILD_VECTORAsBroadCastLoad(BuildVectorSDNode *BVOp,
   if (!IdentitySrc || !BVOp->isOnlyUserOf(IdentitySrc.getNode()))
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heiher wrote:

```suggestion
  if (!IsIdeneity || !IdentitySrc || !BVOp->isOnlyUserOf(IdentitySrc.getNode()))
```

https://github.com/llvm/llvm-project/pull/155960