[llvm] [SelectionDAG] Disable FastISel for swiftasync functions (PR #70741)

[Simon Pilgrim via llvm-commits]

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@@ -1441,11 +1441,21 @@ static void processSingleLocVars(FunctionLoweringInfo &FuncInfo,
   }
 }
 
+static bool shouldEnableFastISel(const Function &Fn) {
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RKSimon wrote:

It's worth a try, it's supposed to allow opt level override at the function level, which is precisely what we're after. 

https://github.com/llvm/llvm-project/pull/70741