[turbofan] Re-separate logic for LET and CONST in BuildVariableAssignment

The structure of this code changed in https://codereview.chromium.org/2201193004,
but the structural changes were not essential to that CL's purpose (which was
to use Variable::binding_needs_init() consistently to decide whether to
hole-check). Now the code should appear as it did before that change, but with
the addition of binding_needs_init() checks.

R=bmeurer@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2237933002
Cr-Commit-Position: refs/heads/master@{#38638}

src/compiler/ast-graph-builder.cc

@@ -3452,6 +3452,15 @@ Node* AstGraphBuilder::BuildVariableAssignment(
         // baseline code might contain debug code that inspects the variable.
         Node* current = environment()->Lookup(variable);
         CHECK_NOT_NULL(current);
+      } else if (mode == LET && op != Token::INIT &&
+                 variable->binding_needs_init()) {
+        // Perform an initialization check for let declared variables.
+        Node* current = environment()->Lookup(variable);
+        if (current->op() == the_hole->op()) {
+          return BuildThrowReferenceError(variable, bailout_id);
+        } else if (current->opcode() == IrOpcode::kPhi) {
+          BuildHoleCheckThenThrow(current, variable, value, bailout_id);
+        }
       } else if (mode == CONST && op == Token::INIT) {
         // Perform an initialization check for const {this} variables.
         // Note that the {this} variable is the only const variable being able
@@ -3460,19 +3469,17 @@ Node* AstGraphBuilder::BuildVariableAssignment(
         if (current->op() != the_hole->op() && variable->is_this()) {
           value = BuildHoleCheckElseThrow(current, variable, value, bailout_id);
         }
-      } else if (IsLexicalVariableMode(mode) && op != Token::INIT) {
-        // Perform an initialization check for lexically declared variables.
-        Node* current = environment()->Lookup(variable);
+      } else if (mode == CONST && op != Token::INIT) {
         if (variable->binding_needs_init()) {
+          Node* current = environment()->Lookup(variable);
           if (current->op() == the_hole->op()) {
             return BuildThrowReferenceError(variable, bailout_id);
           } else if (current->opcode() == IrOpcode::kPhi) {
             BuildHoleCheckThenThrow(current, variable, value, bailout_id);
           }
         }
-        if (mode == CONST) {
-          return BuildThrowConstAssignError(bailout_id);
-        }
+        // Assignment to const is exception in all modes.
+        return BuildThrowConstAssignError(bailout_id);
       }
       environment()->Bind(variable, value);
       return value;
@@ -3487,6 +3494,13 @@ Node* AstGraphBuilder::BuildVariableAssignment(
           return BuildThrowConstAssignError(bailout_id);
         }
         return value;
+      } else if (mode == LET && op != Token::INIT &&
+                 variable->binding_needs_init()) {
+        // Perform an initialization check for let declared variables.
+        const Operator* op =
+            javascript()->LoadContext(depth, variable->index(), false);
+        Node* current = NewNode(op, current_context());
+        value = BuildHoleCheckThenThrow(current, variable, value, bailout_id);
       } else if (mode == CONST && op == Token::INIT) {
         // Perform an initialization check for const {this} variables.
         // Note that the {this} variable is the only const variable being able
@@ -3497,18 +3511,15 @@ Node* AstGraphBuilder::BuildVariableAssignment(
           Node* current = NewNode(op, current_context());
           value = BuildHoleCheckElseThrow(current, variable, value, bailout_id);
         }
-      } else if (IsLexicalVariableMode(mode) && op != Token::INIT) {
-        // Perform an initialization check for lexically declared variables.
+      } else if (mode == CONST && op != Token::INIT) {
         if (variable->binding_needs_init()) {
           const Operator* op =
               javascript()->LoadContext(depth, variable->index(), false);
           Node* current = NewNode(op, current_context());
-          value = BuildHoleCheckThenThrow(current, variable, value, bailout_id);
-        }
-        if (mode == CONST) {
-          // Assignment to const is exception in all modes.
-          return BuildThrowConstAssignError(bailout_id);
+          BuildHoleCheckThenThrow(current, variable, value, bailout_id);
         }
+        // Assignment to const is exception in all modes.
+        return BuildThrowConstAssignError(bailout_id);
       }
       const Operator* op = javascript()->StoreContext(depth, variable->index());
       return NewNode(op, current_context(), value);