[ir] Make `ir::Discard` a child of `ir::Call`.

The `discard` statement is, essentially, a function call. This CL moves
the `ir` node from being under `Instruction` to be under `Call`.

Bug: tint:1905
Change-Id: I8098dfc6047286b1c5249679139bcc83aa1cb21d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/129681
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/ir/call.cc

@@ -18,6 +18,8 @@ TINT_INSTANTIATE_TYPEINFO(tint::ir::Call);
 
 namespace tint::ir {
 
+Call::Call() : Base() {}
+
 Call::Call(uint32_t id, const type::Type* type, utils::VectorRef<Value*> args)
     : Base(id, type), args_(args) {
     for (auto* arg : args) {

src/tint/ir/call.h

@@ -24,11 +24,6 @@ namespace tint::ir {
 /// A Call instruction in the IR.
 class Call : public utils::Castable<Call, Instruction> {
   public:
-    /// Constructor
-    /// @param id the instruction id
-    /// @param type the result type
-    /// @param args the constructor arguments
-    Call(uint32_t id, const type::Type* type, utils::VectorRef<Value*> args);
     Call(const Call& inst) = delete;
     Call(Call&& inst) = delete;
     ~Call() override;
@@ -43,6 +38,15 @@ class Call : public utils::Castable<Call, Instruction> {
     /// @param out the output stream
     void EmitArgs(utils::StringStream& out) const;
 
+  protected:
+    /// Constructor
+    Call();
+    /// Constructor
+    /// @param id the instruction id
+    /// @param type the result type
+    /// @param args the constructor arguments
+    Call(uint32_t id, const type::Type* type, utils::VectorRef<Value*> args);
+
   private:
     utils::Vector<Value*, 1> args_;
 };

src/tint/ir/discard.h

@@ -16,14 +16,14 @@
 #define SRC_TINT_IR_DISCARD_H_
 
 #include "src/tint/debug.h"
-#include "src/tint/ir/instruction.h"
+#include "src/tint/ir/call.h"
 #include "src/tint/utils/castable.h"
 #include "src/tint/utils/string_stream.h"
 
 namespace tint::ir {
 
 /// A discard instruction in the IR.
-class Discard : public utils::Castable<Discard, Instruction> {
+class Discard : public utils::Castable<Discard, Call> {
   public:
     /// Constructor
     Discard();