[Attributor] Clamp operator to extend known state

[uenoku]

Summary

Similar to ^= operator for IntegerState, this patch introduces a += operator to "clamp" known information.

[jdoerfert]

LGTM.

llvm/include/llvm/Transforms/IPO/Attributor.h

struct IntegerState : public AbstractState {
   /// TODO: Consider replacing the operator with a call or using it only when
   ///       we can also take the maximum of the known information, thus when
   ///       \p R is not dependent on additional assumed state.
   IntegerState operator^=(const IntegerState &R) {
     takeAssumedMinimum(R.Assumed);
     return *this;
   }
 
+  /// "Clamp" this state with \p R. The result is the maximum of the known
+  /// information but not more than what was assumed before.
+  IntegerState operator+=(const IntegerState &R) {
+    takeKnownMaximum(R.Known);
+    return *this;
+  }
+
   /// Make this the minimum, known and assumed, of this state and \p R.
   IntegerState operator&=(const IntegerState &R) {
     Known = std::min(Known, R.Known);
     Assumed = std::min(Assumed, R.Assumed);
     return *this;
   }
 
   /// Make this the maximum, known and assumed, of this state and \p R.
struct DerefState : AbstractState {
 
   /// See IntegerState::operator^=
   DerefState operator^=(const DerefState &R) {
     DerefBytesState ^= R.DerefBytesState;
     GlobalState ^= R.GlobalState;
     return *this;
   }
 
+  /// See IntegerState::operator+=
+  DerefState operator+=(const DerefState &R) {
+    DerefBytesState += R.DerefBytesState;
+    GlobalState += R.GlobalState;
+    return *this;
+  }
+
   /// See IntegerState::operator&=
   DerefState operator&=(const DerefState &R) {
     DerefBytesState &= R.DerefBytesState;
     GlobalState &= R.GlobalState;
     return *this;
   }
 
   /// See IntegerState::operator|=