[llvm] [DAGCombiner] Fold trunc(build_vector(ext(x), ext(x)) -> build_vector(x,x) (PR #179857)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Should VectorCombine also be pulling `insertelement (ext v), (ext x), index` -> `ext (insertelement v, x, index)`? cc @RKSimon 

https://github.com/llvm/llvm-project/pull/179857