[llvm] [DAGISel][ARM] Fix vector truncate combine for big-endian (PR #118101)

[Simon Pilgrim via llvm-commits]

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@@ -15495,12 +15495,15 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
       unsigned BuildVecNumElts =  BuildVect.getNumOperands();
       unsigned TruncVecNumElts = VT.getVectorNumElements();
       unsigned TruncEltOffset = BuildVecNumElts / TruncVecNumElts;
+      unsigned FirstElt =
+          DAG.getDataLayout().isBigEndian() ? (TruncEltOffset - 1) : 0;
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RKSimon wrote:

visitTRUNCATE already has isLE - replace this with:
```
unsigned FirstElt =isLE ? 0 : (TruncEltOffset - 1);
```

https://github.com/llvm/llvm-project/pull/118101