[llvm] [DAGCombiner][X86] Correctly clean up high bits in `combinei64TruncSrlAdd` (PR #128353)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> > I was worrying the correctness, but don't know how to prove it. Do you have any idea how to prove the problem of #128309 with alive2?
> 
> We can hardcode c2 into the type and divide anyext into sext+zext (2 pairs): https://alive2.llvm.org/ce/z/7ieYLg

Thanks for the point! I finally proved we must use zext as long as `c1 >= 1 << shamt`: https://alive2.llvm.org/ce/z/7sTBS7. So we cannot save the `movz` instruction.

https://github.com/llvm/llvm-project/pull/128353