[code-assembler] Check that a variable's value is never accessed outside a block.

We should only ever call value() on a variable while we are inside a block.
This CL adds a DEBUG check to this effect.

Bug: 
Change-Id: Ic85fae70e2c3543ff79e3234ba26e1daa234f7e3
Reviewed-on: https://chromium-review.googlesource.com/772233Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49492}

src/compiler/code-assembler.cc

@@ -105,6 +105,8 @@ CodeAssembler::~CodeAssembler() {}
 void CodeAssemblerState::PrintCurrentBlock(std::ostream& os) {
   raw_assembler_->PrintCurrentBlock(os);
 }
+
+bool CodeAssemblerState::InsideBlock() { return raw_assembler_->InsideBlock(); }
 #endif
 
 void CodeAssemblerState::SetInitialDebugInformation(const char* msg,
@@ -1334,11 +1336,14 @@ Node* CodeAssemblerVariable::value() const {
   if (!IsBound()) {
     std::stringstream str;
     str << "#Use of unbound variable:"
+        << "#\n    Variable:      " << *this << "#\n    Current Block: ";
+    state_->PrintCurrentBlock(str);
+    FATAL(str.str().c_str());
+  }
+  if (!state_->InsideBlock()) {
+    std::stringstream str;
+    str << "#Accessing variable value outside a block:"
         << "#\n    Variable:      " << *this;
-    if (state_) {
-      str << "#\n    Current Block: ";
-      state_->PrintCurrentBlock(str);
-    }
     FATAL(str.str().c_str());
   }
 #endif  // DEBUG

src/compiler/code-assembler.h

@@ -1208,6 +1208,7 @@ class V8_EXPORT_PRIVATE CodeAssemblerState {
 
 #if DEBUG
   void PrintCurrentBlock(std::ostream& os);
+  bool InsideBlock();
 #endif  // DEBUG
   void SetInitialDebugInformation(const char* msg, const char* file, int line);
 

src/compiler/raw-machine-assembler.cc

@@ -405,6 +405,8 @@ void RawMachineAssembler::PrintCurrentBlock(std::ostream& os) {
   os << CurrentBlock();
 }
 
+bool RawMachineAssembler::InsideBlock() { return current_block_ != nullptr; }
+
 void RawMachineAssembler::SetInitialDebugInformation(
     AssemblerDebugInfo debug_info) {
   CurrentBlock()->set_debug_info(debug_info);

src/compiler/raw-machine-assembler.h

@@ -831,6 +831,7 @@ class V8_EXPORT_PRIVATE RawMachineAssembler {
   void Bind(RawMachineLabel* label, AssemblerDebugInfo info);
   void SetInitialDebugInformation(AssemblerDebugInfo info);
   void PrintCurrentBlock(std::ostream& os);
+  bool InsideBlock();
 #endif  // DEBUG
 
   // Add success / exception successor blocks and ends the current block ending