[llvm] [WebAssembly] Add assembly support for final EH proposal (PR #107917)

[Heejin Ahn via llvm-commits]

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@@ -147,6 +178,8 @@ let isTerminator = 1, hasSideEffects = 1, isBarrier = 1, hasCtrlDep = 1,
 // usage gets low enough.
 
 // Rethrowing an exception: rethrow
+// The new exnref proposal also uses this instruction as an interim pseudo
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aheejin wrote:

If we want to be strict maybe we can make the old rethrow `RETHROW_LEGACY` or something too and add a new pseudo `RETHROW`, but I didn't feel it very necessary given that this is not very confusing. (In `CATCH`/`CATCH_ALL`'s case it was confusing because in CFGStackify or MCInstLower we have to check `WebAssembly::WasmEnableExnref` many times to check whether the current instruction is the legacy `CATCH` or the new pseudo `CATCH`)

https://github.com/llvm/llvm-project/pull/107917