SILGen: Fudge emitProlog() to emit unreachable later

For some reason, doing it in the other order causes a crash.
I suspect this is because we create new instructions below,
but if there's no insertion point, the instruction is never
added to a basic block.

Author: slavapestov

lib/SILGen/SILGenProlog.cpp

@@ -443,19 +443,6 @@ void SILGenFunction::emitProlog(CaptureInfo captureInfo,
   uint16_t ArgNo = emitProlog(paramList, selfParam, resultType,
                               DC, throws, throwsLoc);
 
-  // Emit an unreachable instruction if a parameter type is
-  // uninhabited
-  if (paramList) {
-    for (auto *param : *paramList) {
-      if (param->getType()->isStructurallyUninhabited()) {
-        SILLocation unreachableLoc(param);
-        unreachableLoc.markAsPrologue();
-        B.createUnreachable(unreachableLoc);
-        break;
-      }
-    }
-  }
-
   // Emit the capture argument variables. These are placed last because they
   // become the first curry level of the SIL function.
   assert((captureInfo.hasBeenComputed() ||
@@ -491,6 +478,19 @@ void SILGenFunction::emitProlog(CaptureInfo captureInfo,
     emitCaptureArguments(*this, DC->getGenericSignatureOfContext(),
                          capture, ++ArgNo);
   }
+
+  // Emit an unreachable instruction if a parameter type is
+  // uninhabited
+  if (paramList) {
+    for (auto *param : *paramList) {
+      if (param->getType()->isStructurallyUninhabited()) {
+        SILLocation unreachableLoc(param);
+        unreachableLoc.markAsPrologue();
+        B.createUnreachable(unreachableLoc);
+        break;
+      }
+    }
+  }
 }
 
 static void emitIndirectResultParameters(SILGenFunction &SGF, Type resultType,

test/SILGen/functions_uninhabited_param.swift

@@ -8,3 +8,10 @@ func foo(baz: Never) -> Int { // expected-note {{'baz' is uninhabited, so this f
 }
 
 func bar(baz: Never) -> Int {} // ok
+
+// We used to crash when emitting the closure below.
+enum E {
+  static func f(_: E) {}
+}
+
+let _: (E.Type) -> (E) -> () = { s in { e in s.f(e) } }