This reapplies r207235 with an additional bugfixes caught by the msan

buildbot - do not insert debug intrinsics before phi nodes.

Debug info for optimized code: Support variables that are on the stack and
described by DBG_VALUEs during their lifetime.

Previously, when a variable was at a FrameIndex for any part of its
lifetime, this would shadow all other DBG_VALUEs and only a single
fbreg location would be emitted, which in fact is only valid for a small
range and not the entire lexical scope of the variable. The included
dbg-value-const-byref testcase demonstrates this.

This patch fixes this by
Local
- emitting dbg.value intrinsics for allocas that are passed by reference
- dropping all dbg.declares (they are now fully lowered to dbg.values)
SelectionDAG
- renamed constructors for SDDbgValue for better readability.
- fix UserValue::match() to handle indirect values correctly
- not inserting an MMI table entries for dbg.values that describe allocas.
- lowering dbg.values that describe allocas into *indirect* DBG_VALUEs.
CodeGenPrepare
- leaving dbg.values for an alloca were they are (see comment)
Other
- regenerated/updated instcombine.ll testcase and included source

rdar://problem/16679879
http://reviews.llvm.org/D3374

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207269 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/Local.cpp

@@ -1037,20 +1037,26 @@ bool llvm::LowerDbgDeclare(Function &F) {
     AllocaInst *AI = dyn_cast_or_null<AllocaInst>(DDI->getAddress());
     // If this is an alloca for a scalar variable, insert a dbg.value
     // at each load and store to the alloca and erase the dbg.declare.
+    // The dbg.values allow tracking a variable even if it is not
+    // stored on the stack, while the dbg.declare can only describe
+    // the stack slot (and at a lexical-scope granularity). Later
+    // passes will attempt to elide the stack slot.
     if (AI && !AI->isArrayAllocation()) {
-
-      // We only remove the dbg.declare intrinsic if all uses are
-      // converted to dbg.value intrinsics.
-      bool RemoveDDI = true;
       for (User *U : AI->users())
         if (StoreInst *SI = dyn_cast<StoreInst>(U))
           ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
         else if (LoadInst *LI = dyn_cast<LoadInst>(U))
           ConvertDebugDeclareToDebugValue(DDI, LI, DIB);
-        else
-          RemoveDDI = false;
-      if (RemoveDDI)
-        DDI->eraseFromParent();
+        else if (CallInst *CI = dyn_cast<CallInst>(U)) {
+	  // This is a call by-value or some other instruction that
+	  // takes a pointer to the variable. Insert a *value*
+	  // intrinsic that describes the alloca.
+	  auto DbgVal =
+	    DIB.insertDbgValueIntrinsic(AI, 0,
+					DIVariable(DDI->getVariable()), CI);
+	  DbgVal->setDebugLoc(DDI->getDebugLoc());
+	}
+      DDI->eraseFromParent();
     }
   }
   return true;