Fuzzer: Handle negative i31s (#6341)

Author: kripken

scripts/fuzz_opt.py

@@ -584,7 +584,7 @@ def fix_double(x):
 
     # JS prints i31 as just a number, so change "i31ref(N)" (which C++ emits)
     # to "N".
-    out = re.sub(r'i31ref\((\d+)\)', r'\1', out)
+    out = re.sub(r'i31ref\((-?\d+)\)', r'\1', out)
 
     lines = out.splitlines()
     for i in range(len(lines)):