Remove unused data member to unbreak -Werror builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279581 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/StackMaps.cpp

@@ -35,8 +35,7 @@ static cl::opt<int> StackMapVersion(
 
 const char *StackMaps::WSMP = "Stack Maps: ";
 
-StackMapOpers::StackMapOpers(const MachineInstr *MI) 
-  : MI(MI) {
+StackMapOpers::StackMapOpers(const MachineInstr *MI) {
   assert(getVarIdx() <= MI->getNumOperands() &&
          "invalid stackmap definition");
 }