[Attributor] Add helper class to compose two structured deduction.

[uenoku]

Summary

This patch introduces a generic way to compose two structured deductions. This will be used for composing generic deduction with MustBeExecutedExplorer and other existing generic deduction.

[jdoerfert]

This is what I wanted but I think I need to see this in action as I'm not a template wizard. (Btw. if you want to clean up the mess I made, feel free!)

[uenoku]

Minor update.

[thakis]

Looks like this fails on Windows: http://45.33.8.238/win/74/step_8.txt

[uenoku]

In D66645#1699907, @thakis wrote:

Looks like this fails on Windows: http://45.33.8.238/win/74/step_8.txt

Thank you for teaching me. A temporary fix is done in rl374086.

llvm/lib/Transforms/IPO/Attributor.cpp

static void clampReturnedValueStates(Attributor &A, const AAType &QueryingAA,
   };
 
   if (!A.checkForAllReturnedValues(CheckReturnValue, QueryingAA))
     S.indicatePessimisticFixpoint();
   else if (T.hasValue())
     S ^= *T;
 }
 
+/// Helper class to compose two generic deduction
+template <typename AAType, typename Base, typename StateType,
+          template <typename...> class F, template <typename...> class G>
+struct AAComposeTwoGenericDeduction
+    : public F<AAType, G<AAType, Base, StateType>, StateType> {
+  AAComposeTwoGenericDeduction(const IRPosition &IRP)
+      : F<AAType, G<AAType, Base, StateType>, StateType>(IRP) {}
+
+  /// See AbstractAttribute::updateImpl(...).
+  ChangeStatus updateImpl(Attributor &A) override {
+    return F<AAType, G<AAType, Base, StateType>, StateType>::updateImpl(A) |
+           G<AAType, Base, StateType>::updateImpl(A);
+  }
+};
+
 /// Helper class for generic deduction: return value -> returned position.
 template <typename AAType, typename Base,
           typename StateType = typename AAType::StateType>
 struct AAReturnedFromReturnedValues : public Base {
   AAReturnedFromReturnedValues(const IRPosition &IRP) : Base(IRP) {}
 
   /// See AbstractAttribute::updateImpl(...).
   ChangeStatus updateImpl(Attributor &A) override {