[bigint] Check some invariants in BigIntVerify().

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I50cf6418f382689559b33b2c5a218435373dec64
Reviewed-on: https://chromium-review.googlesource.com/666920
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48827}

src/objects-debug.cc

@@ -1214,7 +1214,13 @@ void AsyncGeneratorRequest::AsyncGeneratorRequestVerify() {
   next()->ObjectVerify();
 }
 
-void BigInt::BigIntVerify() { CHECK(IsBigInt()); }
+void BigInt::BigIntVerify() {
+  CHECK(IsBigInt());
+  CHECK_GE(length(), 0);
+  CHECK_IMPLIES(is_zero(), !sign());  // There is no -0n.
+  // TODO(neis): Somewhere check that MSD is non-zero. Doesn't hold during some
+  // operations that allocate which is why we can't test it here.
+}
 
 void JSModuleNamespace::JSModuleNamespaceVerify() {
   CHECK(IsJSModuleNamespace());

src/objects/bigint.h

@@ -184,10 +184,8 @@ class V8_EXPORT_PRIVATE BigInt : public HeapObject {
   inline digit_t digit(int n) const;
   inline void set_digit(int n, digit_t value);
 
-  bool is_zero() {
-    DCHECK(length() > 0 || !sign());  // There is no -0n.
-    return length() == 0;
-  }
+  bool is_zero() const { return length() == 0; }
+
   static const int kBitfieldOffset = HeapObject::kHeaderSize;
   static const int kDigitsOffset = kBitfieldOffset + kPointerSize;
   static const int kHeaderSize = kDigitsOffset;