Don't duplicate workgroup for derivatives

* Because SPIR-V is SSA, we can use the current workgroup for all cross-group
  operations, knowing that the output ID of any such opcode can't also be the
  input (so doing workgroup operations one-at-a-time is still fine, we won't
  corrupt results of later threads by updating earlier ones.

renderdoc/driver/shaders/spirv/spirv_debug_setup.cpp

@@ -590,10 +590,6 @@ rdcarray<ShaderDebugState> Debugger::ContinueDebug()
   if(active.Finished())
     return ret;
 
-  rdcarray<DenseIdMap<ShaderVariable>> oldworkgroup;
-
-  oldworkgroup.resize(workgroup.size());
-
   rdcarray<bool> activeMask;
 
   // do 100 in a chunk
@@ -602,12 +598,6 @@ rdcarray<ShaderDebugState> Debugger::ContinueDebug()
     if(active.Finished())
       break;
 
-    // set up the old workgroup so that cross-workgroup/cross-quad operations (e.g. DDX/DDY) get
-    // consistent results even when we step the quad out of order. Otherwise if an operation reads
-    // and writes from the same register we'd trash data needed for other workgroup elements.
-    for(size_t i = 0; i < oldworkgroup.size(); i++)
-      oldworkgroup[i] = workgroup[i].ids;
-
     // calculate the current mask of which threads are active
     CalcActiveMask(activeMask);
 
@@ -653,7 +643,7 @@ rdcarray<ShaderDebugState> Debugger::ContinueDebug()
             l++;
           }
 
-          thread.StepNext(&state, oldworkgroup);
+          thread.StepNext(&state, workgroup);
           state.stepIndex = steps;
           state.sourceVars = thread.sourceVars;
           thread.FillCallstack(state);
@@ -661,7 +651,7 @@ rdcarray<ShaderDebugState> Debugger::ContinueDebug()
         }
         else
         {
-          thread.StepNext(NULL, oldworkgroup);
+          thread.StepNext(NULL, workgroup);
         }
       }
     }