[NFC][AMDGPU] Default initialize the Subtarget

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)

Differential Revision: https://reviews.llvm.org/D154414

Author: jchlanda

llvm/lib/Target/AMDGPU/R600ISelDAGToDAG.cpp

@@ -20,7 +20,7 @@
 
 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);