[llvm] [SelectionDAG] Not issue TRAP node if naked function (PR #132147)

[Heejin Ahn via llvm-commits]

aheejin wrote:

Sorry for the late comment. I don't think we should do this for WebAssembly. See the comments here:
https://github.com/llvm/llvm-project/blob/58a0c9570c69ecdf23e998637d2b82cfa455bf14/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp#L132-L135

So if we change the return type of `naked` to i32,
```ll
define dso_local i32 @naked() naked "frame-pointer"="all" {
  call void @main()
  unreachable
}
```

The generated `.s` or `.o` will not be valid.
```console

$ llc -mtriple wasm32 naked-fn-with-frame-pointer.ll

$ llvm-mc -triple=wasm32-unknown-unknown  naked-fn-with-frame-pointer.s
...
naked-fn-with-frame-pointer.s:12:2: error: type mismatch, expected [i32] but got []
        end_function
        ^
...
```
The reason is, whether the function returns or not, Wasm's validation rule ensures the function's return type matches the type of the value left on the Wasm value stack at the function end (or anytime there is a `return` instruction). And here `naked`'s return type is changed to `i32` but the value stack does not have an `i32` on it. But if we have an `unreachable` instruction at the end, the stack become _polymorphic_, which allows any types of values be popped. (See https://webassembly.github.io/spec/core/valid/instructions.html about polymorphic stack)

I think this PR should have an exception for Wasm target.

https://github.com/llvm/llvm-project/pull/132147