Move getCommonSubClass() into TRI.

It will soon need the context.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140896 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveStackAnalysis.h

@@ -25,6 +25,8 @@
 namespace llvm {
 
   class LiveStacks : public MachineFunctionPass {
+    const TargetRegisterInfo *TRI;
+
     /// Special pool allocator for VNInfo's (LiveInterval val#).
     ///
     VNInfo::Allocator VNInfoAllocator;