[bigint] Fix asIntN/asUintN for n == kMaxLengthBits

And also ensure that it doesn't allocate when that's avoidable.

Bug: v8:6791
Change-Id: Ied2d2c9c5718aed9839f17739d743353cbadee88
Reviewed-on: https://chromium-review.googlesource.com/786170Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49683}

src/builtins/builtins-bigint.cc

@@ -77,7 +77,7 @@ BUILTIN(BigIntAsUintN) {
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, bigint,
                                      BigInt::FromObject(isolate, bigint_obj));
 
-  return *BigInt::AsUintN(bits->Number(), bigint);
+  RETURN_RESULT_OR_FAILURE(isolate, BigInt::AsUintN(bits->Number(), bigint));
 }
 
 BUILTIN(BigIntAsIntN) {

src/objects/bigint.h

@@ -138,7 +138,7 @@ class V8_EXPORT_PRIVATE BigInt : public BigIntBase {
   static ComparisonResult CompareToDouble(Handle<BigInt> x, double y);
 
   static Handle<BigInt> AsIntN(uint64_t n, Handle<BigInt> x);
-  static Handle<BigInt> AsUintN(uint64_t n, Handle<BigInt> x);
+  static MaybeHandle<BigInt> AsUintN(uint64_t n, Handle<BigInt> x);
 
   DECL_CAST(BigInt)
   DECL_VERIFIER(BigInt)