[AMDGPU] Always enable XNACK feature when support is explicitly requested

Differential Revision: https://reviews.llvm.org/D74630

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

@@ -150,7 +150,9 @@ GCNSubtarget::initializeSubtargetDependencies(const Triple &TT,
 
   HasFminFmaxLegacy = getGeneration() < AMDGPUSubtarget::VOLCANIC_ISLANDS;
 
-  if (DoesNotSupportXNACK && EnableXNACK) {
+  // Disable XNACK on targets where it is not enabled by default unless it is
+  // explicitly requested.
+  if (!FS.contains("+xnack") && DoesNotSupportXNACK && EnableXNACK) {
     ToggleFeature(AMDGPU::FeatureXNACK);
     EnableXNACK = false;
   }