ast: Move [Set]IsSwizzle() from IdentifierExpression to MemberAccessorExpression

This is more a property of a MemberAccessorExpression than the identifier itself.

Change-Id: Icb17df1fe43a959332d73df026e77ca4e07d23ed
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/40140
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/ast/identifier_expression.h

@@ -39,12 +39,6 @@ class IdentifierExpression : public Castable<IdentifierExpression, Expression> {
   /// @returns the symbol for the identifier
   Symbol symbol() const { return sym_; }
 
-  /// Sets the identifier as a swizzle
-  void SetIsSwizzle() { is_swizzle_ = true; }
-
-  /// @returns true if this is a swizzle identifier
-  bool IsSwizzle() const { return is_swizzle_; }
-
   /// Clones this node and all transitive child nodes using the `CloneContext`
   /// `ctx`.
   /// @note Semantic information such as resolved expression type and intrinsic
@@ -68,8 +62,6 @@ class IdentifierExpression : public Castable<IdentifierExpression, Expression> {
   IdentifierExpression(const IdentifierExpression&) = delete;
 
   Symbol const sym_;
-
-  bool is_swizzle_ = false;  // Semantic info
 };
 
 }  // namespace ast

src/ast/member_accessor_expression.h

@@ -45,6 +45,12 @@ class MemberAccessorExpression
   /// @returns the member expression
   IdentifierExpression* member() const { return member_; }
 
+  /// Sets the identifier as a swizzle
+  void SetIsSwizzle() { is_swizzle_ = true; }
+
+  /// @returns true if this is a swizzle identifier
+  bool IsSwizzle() const { return is_swizzle_; }
+
   /// Clones this node and all transitive child nodes using the `CloneContext`
   /// `ctx`.
   /// @note Semantic information such as resolved expression type and intrinsic
@@ -69,6 +75,8 @@ class MemberAccessorExpression
 
   Expression* const struct_;
   IdentifierExpression* const member_;
+
+  bool is_swizzle_ = false;  // Semantic info
 };
 
 }  // namespace ast