[NFC][AMDGPU] Default initialize the Subtarget

[jchlanda]

Summary

This is to address a static analizer warning:

The pointer field will point to an arbitrary memory location, any attempt to write may cause corruption.
In «unnamed> R600DAGToDAGISel.:R600DAGToDAGISel (llvm::TargetMachine &, livm::CodeGenOpt::Level): A pointer field is not initialized in the constructor (CWE-457)

[nickdesaulniers]

Thanks for the patch. Consider adding the output from the static analyzer to the commit description (in phab and git) for this patch and in the future.

[jchlanda]

In D154414#4481856, @nickdesaulniers wrote:

Thanks for the patch. Consider adding the output from the static analyzer to the commit description (in phab and git) for this patch and in the future.

Sure, updated the summary and will add it to the commit message. Thanks.

llvm/lib/Target/AMDGPU/R600ISelDAGToDAG.cpp

 #include "AMDGPUISelDAGToDAG.h"
 #include "MCTargetDesc/R600MCTargetDesc.h"
 #include "R600.h"
 #include "R600Subtarget.h"
 #include "llvm/Analysis/ValueTracking.h"
 
 namespace {
 class R600DAGToDAGISel : public AMDGPUDAGToDAGISel {
-  const R600Subtarget *Subtarget;
+  const R600Subtarget *Subtarget = nullptr;
 
   bool isConstantLoad(const MemSDNode *N, int cbID) const;
   bool SelectGlobalValueConstantOffset(SDValue Addr, SDValue &IntPtr);
   bool SelectGlobalValueVariableOffset(SDValue Addr, SDValue &BaseReg,
                                        SDValue &Offset);
 
 public:
   R600DAGToDAGISel() = delete;