[llvm] [LoongArch] Optimize extractelement containing variable index for lsx (PR #156792)

[via llvm-commits]

zhaoqi5 wrote:

> ...
> The result shows that the tests (except for `Alloc.s`) are same as shown in the link. I am not sure why this happens.

According to test `GCC-C-execute-20000808-1`, if `extract_vector_elt` is `Legal`, `DAGCombiner::visitEXTRACT_VECTOR_ELT` can transform `extract_vector_elt(vector_shuffle)` to `extract_vector_elt`. If `Custom`, the transformation will not happen.

https://github.com/llvm/llvm-project/pull/156792