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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> A counterexample for original implementation: https://alive2.llvm.org/ce/z/YowPZY
> We should keep low `64 - shamt` bits instead of `shamt - 32`.
> Proof: https://alive2.llvm.org/ce/z/z_jdHD

The alive2 just proves the problem in calculation of NewAddConstVal, but is the problem in #128309 actually caused by zext vs. anyext?

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