[llvm] [InstCombine] Pull extract through broadcast (PR #143380)

[via llvm-commits]

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@@ -104,10 +101,7 @@ define i32 @extractelement_shuffle_maybe_out_of_range(i32 %v) {
 
 define i32 @extractelement_shuffle_invalid_index(i32 %v) {
 ; CHECK-LABEL: @extractelement_shuffle_invalid_index(
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agorenstein-nvidia wrote:

Here we optimize, but also in doing so miss the `poison` "opportunity" (in that the illegal -1 index-usage is removed). So I believe this is legal, but perhaps not optimal. However, I'm a bit confused: it is the case that earlier in our transformation we explicitly look for invalid indices in `extractelement`s:

```
    // InstSimplify should handle cases where the index is invalid.
    // For fixed-length vector, it's invalid to extract out-of-range element.
```

Without tracing through, it looks like this may "just" be an order-of-transformations issue. I'm inclined to preserve the behavior. I've simply changed the test to reflect the new output; is there a preferred way of updating/changing these tests in this sort of situation?

https://github.com/llvm/llvm-project/pull/143380