Bug 1685708 - Part 1: Handle no-op cases for 64-bit results in Big.as{Uint,Int}N. r=jandem

The fast path didn't handle the no-op case, which actually made the fast-path
slower for the case when no conversion is necessary.

Differential Revision: https://phabricator.services.mozilla.com/D101167

js/src/vm/BigIntType.cpp

@@ -2588,7 +2588,11 @@ BigInt* BigInt::asUintN(JSContext* cx, HandleBigInt x, uint64_t bits) {
   if (bits <= 64) {
     uint64_t u64 = toUint64(x);
     uint64_t mask = uint64_t(-1) >> (64 - bits);
-    return createFromUint64(cx, u64 & mask);
+    uint64_t n = u64 & mask;
+    if (u64 == n && x->absFitsInUint64()) {
+      return x;
+    }
+    return createFromUint64(cx, n);
   }
 
   if (bits >= MaxBitLength) {
@@ -2645,7 +2649,11 @@ BigInt* BigInt::asIntN(JSContext* cx, HandleBigInt x, uint64_t bits) {
   }
 
   if (bits == 64) {
-    return createFromInt64(cx, toInt64(x));
+    int64_t n = toInt64(x);
+    if (((n < 0) == x->isNegative()) && x->absFitsInUint64()) {
+      return x;
+    }
+    return createFromInt64(cx, n);
   }
 
   if (bits > MaxBitLength) {