resolver: Migrate validation to behavior analysis

Migrate some of the validation logic over to use the results of behavior
analysis.

The most significant changes are:
* Unreachable-statements now consider merge-points of control flow. For
  example, if all branches of a if-statement or switch-statement either
  return or discard, the next statement will be considered unreachable.
* Unreachable statements are no longer an error, but a warning. See
  https://github.com/gpuweb/gpuweb/issues/2378.
* Statements that follow a loops that does not break, or have a
  conditional will now be considered unreachable.
* Unreachable statements produced by the SPIR-V reader are now removed
  using the new RemoveUnreachableStatements transform.

Some other new changes include additional validation for the continuing
block for for-loops, to match the rules of a loop continuing block.
The new cases this validation is testing for are not expressible in
WGSL, but some transforms may produce complex continuing statements that
might violate these rules. All the writers are able to decay these
complex for-loop continuing statements to regular loops.

Bug: tint:1302
Change-Id: I0d8a48c73d5d5c30a1cddf92cc3383a692a58e61
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/71500
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/sem/for_loop_statement.cc

@@ -28,5 +28,9 @@ ForLoopStatement::ForLoopStatement(const ast::ForLoopStatement* declaration,
 
 ForLoopStatement::~ForLoopStatement() = default;
 
+const ast::ForLoopStatement* ForLoopStatement::Declaration() const {
+  return static_cast<const ast::ForLoopStatement*>(Base::Declaration());
+}
+
 }  // namespace sem
 }  // namespace tint

src/sem/for_loop_statement.h

@@ -43,6 +43,9 @@ class ForLoopStatement : public Castable<ForLoopStatement, CompoundStatement> {
   /// Destructor
   ~ForLoopStatement() override;
 
+  /// @returns the AST node
+  const ast::ForLoopStatement* Declaration() const;
+
   /// @returns the for-loop condition expression
   const Expression* Condition() const { return condition_; }
 

src/sem/statement.h

@@ -106,11 +106,20 @@ class Statement : public Castable<Statement, Node> {
   /// @return the behaviors of this statement
   sem::Behaviors& Behaviors() { return behaviors_; }
 
+  /// @returns true if this statement is reachable by control flow according to
+  /// the behavior analysis
+  bool IsReachable() const { return is_reachable_; }
+
+  /// @param is_reachable whether this statement is reachable by control flow
+  /// according to the behavior analysis
+  void SetIsReachable(bool is_reachable) { is_reachable_ = is_reachable; }
+
  private:
   const ast::Statement* const declaration_;
   const CompoundStatement* const parent_;
   const sem::Function* const function_;
   sem::Behaviors behaviors_{sem::Behavior::kNext};
+  bool is_reachable_ = true;
 };
 
 /// CompoundStatement is the base class of statements that can hold other