[llvm] [X86][WinEH] Restore parent RSP before Win64 catchret continuation (PR #191616)

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taigacon wrote:

Thanks, that makes sense. I reworked the patch pretty substantially based on that comment. It now carries the original dynalloca size through DYNAMIC_STACKALLOC, introduces a dedicated WIN_ALLOCA lowering for x86_64-windows-msvc, and handles the Win64-specific stack behavior there.

I also updated the affected Win64 tests for the new stack shape. For some of them I tried constructing small C++ examples and comparing against cl output to sanity-check the expected assembly. This area is honestly pretty tricky, so the code is not especially pretty, but hopefully the direction is better now. Please let me know if you still see anything wrong.

https://github.com/llvm/llvm-project/pull/191616