[llvm] [IR] Add @llvm.structured.alloca (PR #186811)

[Nathan Gauër via llvm-commits]

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Keenuts wrote:

I think we should a target-dependent alignment. Having an explicit alignment would not be lower-able to logical SPIR-V for alloca. I'll add a note in the docs

https://github.com/llvm/llvm-project/pull/186811