IRGen: Fix the insert point after inserting the dynamic replacement prolog

The code used the last basic block in the function instead of the last
basic block created for the dynamic replacement prolog.

rdar://77073666

Author: aschwaighofer

lib/IRGen/IRGenSIL.cpp

@@ -1958,7 +1958,7 @@ static void emitEntryPointArgumentsNativeCC(IRGenSILFunction &IGF,
     if (IGF.CurSILFn->isDynamicallyReplaceable()) {
       IGF.IGM.createReplaceableProlog(IGF, IGF.CurSILFn);
       // Remap the entry block.
-      IGF.LoweredBBs[&*IGF.CurSILFn->begin()] = LoweredBB(&IGF.CurFn->back(), {});
+      IGF.LoweredBBs[&*IGF.CurSILFn->begin()] = LoweredBB(IGF.Builder.GetInsertBlock(), {});
     }
   }
 

test/Interpreter/async_dynamic.swift

@@ -12,8 +12,29 @@ public dynamic func number() async -> Int {
     return 100
 }
 
+enum SomeError : Error {
+  case err
+}
+
+class C {
+  dynamic func a() async throws -> Int? {
+    return 0
+  }
+
+  dynamic func b() async throws {
+    guard let data = try await a() else {
+      throw SomeError.err
+    }
+  }
+
+}
 @main struct Main {
+
   static func main() async {
+    do {
+      try await C().b()
+    } catch _ { assertionFailure("should not throw") }
+
     // CHECK: 100
     let value = await number()
     print(value)