AMDGPU: skip AMDGPUAttributor and AMDGPUImageIntrinsicOptimizerPass on R600. (#162207)

These passes call `getSubtarget<GCNSubtarget>`, which doesn't work on
R600 targets, as that uses an `R600Subtarget` type, instead.
Unfortunately, `TargetMachine::getSubtarget<ST>` does an unchecked
static_cast to `ST&`, which makes it easy for this error to go
undetected.

The modifications here were verified by running check-llvm with an
assert added to getSubtarget. However, that asssert requires that RTTI
is enabled, which LLVM doesn't use, so I've reverted the assert before
sending this fix upstream.

These errors have been present for some time, but were detected after
#162040 caused an uninitialized memory read to be reported by asan/msan.

Author: jyknight

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

@@ -929,8 +929,10 @@ void AMDGPUTargetMachine::registerPassBuilderCallbacks(PassBuilder &PB) {
                                             ThinOrFullLTOPhase Phase) {
     if (Level != OptimizationLevel::O0) {
       if (!isLTOPreLink(Phase)) {
-        AMDGPUAttributorOptions Opts;
-        MPM.addPass(AMDGPUAttributorPass(*this, Opts, Phase));
+        if (getTargetTriple().isAMDGCN()) {
+          AMDGPUAttributorOptions Opts;
+          MPM.addPass(AMDGPUAttributorPass(*this, Opts, Phase));
+        }
       }
     }
   });
@@ -1296,7 +1298,8 @@ void AMDGPUPassConfig::addIRPasses() {
   if (LowerCtorDtor)
     addPass(createAMDGPUCtorDtorLoweringLegacyPass());
 
-  if (isPassEnabled(EnableImageIntrinsicOptimizer))
+  if (TM.getTargetTriple().isAMDGCN() &&
+      isPassEnabled(EnableImageIntrinsicOptimizer))
     addPass(createAMDGPUImageIntrinsicOptimizerPass(&TM));
 
   // This can be disabled by passing ::Disable here or on the command line