[llvm] [GlobalIsel] Pust cast through build vector (PR #104634)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

If I read it correctly, the DAG is doing the push only for trunc:
https://github.com/llvm/llvm-project/blob/80c51fad3b683c8c9ee4c26aa41eda2f78a29627/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L15033

https://github.com/llvm/llvm-project/pull/104634