[llvm] [GlobalIsel] Combine trunc of binop (PR #107721)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Instcombine is more aggressive in this combine:
https://github.com/llvm/llvm-project/blob/71560771d73d71e3ac9b73c81496efecc2e3a08d/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L544

https://github.com/llvm/llvm-project/pull/107721