[bigint] Fix early-return in asIntN

For "top digit" (of the result) comparison to be applicable, we must
also check that there are no further digits in the source.

Bug: v8:7150
Change-Id: I6ad317f6f600e11fef59b9907da1055e5586a3a8
Reviewed-on: https://chromium-review.googlesource.com/804639Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49846}

src/objects/bigint.cc

@@ -1928,12 +1928,13 @@ Handle<BigInt> BigInt::AsIntN(uint64_t n, Handle<BigInt> x) {
   if (x->is_zero()) return x;
   if (n == 0) return MutableBigInt::Zero(x->GetIsolate());
   uint64_t needed_length = (n + kDigitBits - 1) / kDigitBits;
+  uint64_t x_length = static_cast<uint64_t>(x->length());
   // If {x} has less than {n} bits, return it directly.
-  if (static_cast<uint64_t>(x->length()) < needed_length) return x;
+  if (x_length < needed_length) return x;
   DCHECK_LE(needed_length, kMaxInt);
   digit_t top_digit = x->digit(static_cast<int>(needed_length) - 1);
   digit_t compare_digit = static_cast<digit_t>(1) << ((n - 1) % kDigitBits);
-  if (top_digit < compare_digit) return x;
+  if (x_length == needed_length && top_digit < compare_digit) return x;
   // Otherwise we have to truncate (which is a no-op in the special case
   // of x == -2^(n-1)), and determine the right sign. We also might have
   // to subtract from 2^n to simulate having two's complement representation.

test/mjsunit/harmony/bigint/as-int-n.js

@@ -145,6 +145,8 @@
 }{
   assertThrows(() => BigInt.asIntN(3, 12), TypeError);
   assertEquals(-4n, BigInt.asIntN(3, "12"));
+  assertEquals(0x123456789abcdefn,
+               BigInt.asIntN(64, 0xabcdef0123456789abcdefn));
 }
 
 // BigInt.asUintN