[Attributor] Add support for compound assignment for ChangeStatus

A common use of `ChangeStatus` is as follows:
```
ChangeStatus Changed = ChangeStatus::UNCHANGED;
Changed |= foo();
```
where `foo` returns `ChangeStatus` as well. Currently `ChangeStatus` doesn't
support compound assignment, we have to write as
```
Changed = Changed | foo();
```
which is not that convenient.

This patch add the support for compound assignment for `ChangeStatus`. Compound
assignment is usually implemented as a member function, and binary arithmetic
operator is therefore implemented using compound assignment. However, unlike
regular C++ class, enum class doesn't support member functions. As a result, they
can only be implemented in the way shown in the patch.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106109

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

@@ -177,7 +177,9 @@ enum class ChangeStatus {
 };
 
 ChangeStatus operator|(ChangeStatus l, ChangeStatus r);
+ChangeStatus &operator|=(ChangeStatus &l, ChangeStatus r);
 ChangeStatus operator&(ChangeStatus l, ChangeStatus r);
+ChangeStatus &operator&=(ChangeStatus &l, ChangeStatus r);
 
 enum class DepClassTy {
   REQUIRED, ///< The target cannot be valid if the source is not.

llvm/lib/Transforms/IPO/Attributor.cpp

@@ -161,9 +161,17 @@ static cl::opt<bool>
 ChangeStatus llvm::operator|(ChangeStatus L, ChangeStatus R) {
   return L == ChangeStatus::CHANGED ? L : R;
 }
+ChangeStatus &llvm::operator|=(ChangeStatus &L, ChangeStatus R) {
+  L = L | R;
+  return L;
+}
 ChangeStatus llvm::operator&(ChangeStatus L, ChangeStatus R) {
   return L == ChangeStatus::UNCHANGED ? L : R;
 }
+ChangeStatus &llvm::operator&=(ChangeStatus &L, ChangeStatus R) {
+  L = L & R;
+  return L;
+}
 ///}
 
 bool AA::isValidInScope(const Value &V, const Function *Scope) {

llvm/lib/Transforms/IPO/OpenMPOpt.cpp

@@ -3505,7 +3505,7 @@ struct AAFoldRuntimeCallCallSiteReturned : AAFoldRuntimeCall {
 
     switch (RFKind) {
     case OMPRTL___kmpc_is_spmd_exec_mode:
-      Changed = Changed | foldIsSPMDExecMode(A);
+      Changed |= foldIsSPMDExecMode(A);
       break;
     default:
       llvm_unreachable("Unhandled OpenMP runtime function!");