Fuzzer: Fix JS printing of large BigInts (#8445)

`Number(big) | 0` was wrong, as the Number conversion applies JS
rounding when it turns i64 into double. To fix this, get the bits we want first
using a precise i64 operation on BigInts.

Author: kripken

scripts/fuzz_shell.js

@@ -143,7 +143,7 @@ function printed(x, y) {
   } else if (typeof x === 'bigint') {
     // Print bigints in legalized form, which is two 32-bit numbers of the low
     // and high bits.
-    return (Number(x) | 0) + ' ' + (Number(x >> 32n) | 0)
+    return (Number(x & 0xffffffffn) | 0) + ' ' + (Number(x >> 32n) | 0)
   } else if (typeof x !== 'number') {
     // Something that is not a number or string, like a reference. We can't
     // print a reference because it could look different after opts - imagine

test/lit/node/fuzz_shell_bigint.wast

@@ -0,0 +1,22 @@
+;; Test that bigints are printed correctly in JS, both the low and high bits.
+
+(module
+  (func $big (export "big") (result i64)
+    (i64.const -1)
+  )
+
+  (func $medium (export "medium") (result i64)
+    ;; A number big enough to hit the JS 2^53 precision limit. We must print it
+    ;; carefully.
+    (i64.const 0x1000000000000001)
+  )
+)
+
+;; RUN: wasm-opt %s -o %t.wasm -q
+;; RUN: node %S/../../../scripts/fuzz_shell.js %t.wasm | filecheck %s
+
+;; CHECK:      [fuzz-exec] calling big
+;; CHECK-NEXT: [fuzz-exec] note result: big => -1 -1
+;; CHECK-NEXT: [fuzz-exec] calling medium
+;; CHECK-NEXT: [fuzz-exec] note result: medium => 1 268435456
+