[clang][Interp] Classify correct type for compound shifts

RT must be the type of the RHS, otherwise this later fails when
we set the RHS value to the temporary variable.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -1468,7 +1468,7 @@ bool ByteCodeExprGen<Emitter>::VisitCompoundAssignOperator(
   std::optional<PrimType> LHSComputationT =
       classify(E->getComputationLHSType());
   std::optional<PrimType> LT = classify(LHS->getType());
-  std::optional<PrimType> RT = classify(E->getComputationResultType());
+  std::optional<PrimType> RT = classify(RHS->getType());
   std::optional<PrimType> ResultT = classify(E->getType());
 
   if (!LT || !RT || !ResultT || !LHSComputationT)

clang/test/AST/Interp/shifts.cpp

@@ -188,3 +188,12 @@ namespace shifts {
                           // ref-cxx17-error {{not an integral constant expression}} \
                           // ref-cxx17-note {{in call to 'foo(2)'}}
 };
+
+namespace LongInt {
+  constexpr int f() {
+    int a = 1;
+    a <<= (long)0;
+    return 1;
+  }
+  static_assert(f() == 1, "");
+};