[InstrProf] Mark counters as used in debug correlation mode

In debug info correlation mode we do not emit the data globals so we
need to explicitly mark the counter globals as used so they don't get
stripped.

Reviewed By: kyulee

Differential Revision: https://reviews.llvm.org/D115981

Author: ellishg

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

@@ -997,8 +997,11 @@ InstrProfiling::getOrCreateRegionCounters(InstrProfIncrementInst *Inc) {
         ConstantExpr::getBitCast(ValuesVar, Type::getInt8PtrTy(Ctx));
   }
 
-  if (DebugInfoCorrelate)
+  if (DebugInfoCorrelate) {
+    // Mark the counter variable as used so that it isn't optimized out.
+    CompilerUsedVars.push_back(PD.RegionCounters);
     return PD.RegionCounters;
+  }
 
   // Create data variable.
   auto *IntPtrTy = M->getDataLayout().getIntPtrType(M->getContext());