[dxbc] Fix index returned by ImmAtomicConsume instruction

Since the atomic operations always return the old value, we have to
subtract one for the consume instruction. The append instruction is
unaffected. Fixes an issue with vegetation in Final Fantasy XV.

src/dxbc/dxbc_compiler.cpp

@@ -2238,6 +2238,8 @@ namespace dxvk {
       case DxbcOpcode::ImmAtomicConsume:
         value.id = m_module.opAtomicISub(typeId, ptrId,
           scopeId, semanticsId, m_module.constu32(1));
+        value.id = m_module.opISub(typeId, value.id,
+          m_module.constu32(1));
         break;
       
       default: