[llvm] Skip stack protectors on alloca's which have new metadata to opt out (PR #170229)

[Nikita Popov via llvm-commits]

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nikic wrote:

```suggestion
alloca instruction with this metadata and value `i32 0` will be skipped when deciding whether
```
or something.

This is clarified later, but confusing on first read.

https://github.com/llvm/llvm-project/pull/170229