[ir] Validate loop results

This fixes a fuzzer issue that was using an instruction result for
multiple instructions.

Fixed: 422822304
Change-Id: I5bfdc578a9dc79e5f4f3af128a04fd284c740238
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/245698
Reviewed-by: dan sinclair <[email protected]>
Commit-Queue: dan sinclair <[email protected]>
Auto-Submit: James Price <[email protected]>

Author: jrprice

src/tint/lang/core/ir/validator.cc

@@ -3340,6 +3340,8 @@ void Validator::CheckIf(const If* if_) {
 }
 
 void Validator::CheckLoop(const Loop* l) {
+    CheckResults(l);
+
     // Note: Tasks are queued in reverse order of their execution
     tasks_.Push([this, l] {
         first_continues_.Remove(l);  // No need for this any more. Free memory.

src/tint/lang/core/ir/validator_flow_control_test.cc

@@ -326,6 +326,25 @@ TEST_F(IR_ValidatorTest, Loop_EmptyBody) {
 )")) << res.Failure();
 }
 
+TEST_F(IR_ValidatorTest, Loop_NullResult) {
+    auto* f = b.Function("my_func", ty.void_());
+
+    auto* loop = b.Loop();
+    loop->Body()->Append(b.Return(f));
+
+    loop->SetResults(Vector<InstructionResult*, 1>{nullptr});
+
+    f->Block()->Append(loop);
+    f->Block()->Append(b.Return(f));
+
+    auto res = ir::Validate(mod);
+    ASSERT_NE(res, Success);
+    EXPECT_THAT(res.Failure().reason, testing::HasSubstr(R"(:3:5 error: loop: result is undefined
+    undef = loop [b: $B2] {  # loop_1
+    ^^^^^
+)")) << res.Failure();
+}
+
 TEST_F(IR_ValidatorTest, Switch_RootBlock) {
     auto* switch_ = b.Switch(1_i);
     auto* def = b.DefaultCase(switch_);