[llvm] [SandboxIR] Add debug checker to compare IR before/after a revert (PR #115968)

[via llvm-commits]

================
@@ -1844,3 +1844,41 @@ define void @foo(i32 %arg, float %farg) {
   Ctx.revert();
   EXPECT_FALSE(FAdd->getFastMathFlags() != OrigFMF);
 }
+
+TEST_F(TrackerTest, IRSnapshotCheckerNoChanges) {
+  parseIR(C, R"IR(
+define i32 @foo(i32 %arg) {
+  %add0 = add i32 %arg, %arg
+  ret i32 %add0
+}
+)IR");
+  Function &LLVMF = *M->getFunction("foo");
+  sandboxir::Context Ctx(C);
+
+  [[maybe_unused]] auto *F = Ctx.createFunction(&LLVMF);
+  sandboxir::IRSnapshotChecker Checker(Ctx);
+  Checker.save();
+  Checker.expectNoDiff();
+}
+
+TEST_F(TrackerTest, IRSnapshotCheckerDiesWithUnexpectedChanges) {
+  parseIR(C, R"IR(
+define i32 @foo(i32 %arg) {
+  %add0 = add i32 %arg, %arg
+  %add1 = add i32 %add0, %arg
+  ret i32 %add1
+}
+)IR");
+  Function &LLVMF = *M->getFunction("foo");
+  sandboxir::Context Ctx(C);
+
+  auto *F = Ctx.createFunction(&LLVMF);
+  auto *BB = &*F->begin();
+  auto It = BB->begin();
+  sandboxir::Instruction *Add0 = &*It++;
+  sandboxir::Instruction *Add1 = &*It++;
+  sandboxir::IRSnapshotChecker Checker(Ctx);
+  Checker.save();
+  Add1->setOperand(1, Add0);
+  EXPECT_DEATH(Checker.expectNoDiff(), "Found IR difference");
+}
----------------
vporpo wrote:

perhaps we should also exercise running `save()` again (or reset() + save()) after some changes are made?

https://github.com/llvm/llvm-project/pull/115968