[llvm] [SelectionDAG] Freeze dynamic vector indices lowered through the stack (PR #225031)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

we need to get this in soon to address the miscompile - please can you add the aarch64 regression test pr88959.ll from #88985

https://github.com/llvm/llvm-project/pull/225031