Fix update_mir_test_checks.py to run on python3

[RKSimon]

Summary

Split off from D58817

[MaskRay]

utils/update_mir_test_checks.py

413

encode() should also work if we don't expect Unicode in l. But it just saves a few characters so may not be worthy

utils/update_mir_test_checks.py

def __init__(self, bin):
         self.bin = bin
 
     def __call__(self, args, ir):
         if ir.endswith('.mir'):
             args = '{} -x mir'.format(args)
         with open(ir) as ir_file:
             stdout = subprocess.check_output('{} {}'.format(self.bin, args),
                                              shell=True, stdin=ir_file)
+            if sys.version_info[0] > 2:
+              stdout = stdout.decode()
             # Fix line endings to unix CR style.
             stdout = stdout.replace('\r\n', '\n')
         return stdout
 
 
 class Run:
     def __init__(self, prefixes, cmd_args, triple):
         self.prefixes = prefixes
for input_line in input_lines:
                 func_name = None
             if should_add_line_to_output(input_line, prefix_set):
                 output_lines.append(input_line)
 
 
     log('Writing {} lines to {}...'.format(len(output_lines), test), verbose)
 
     with open(test, 'wb') as fd:
-        fd.writelines([l + '\n' for l in output_lines])
+        fd.writelines(['{}\n'.format(l).encode('utf-8') for l in output_lines])
 
 
 def main():
     parser = argparse.ArgumentParser(
         description=__doc__, formatter_class=argparse.RawTextHelpFormatter)
     parser.add_argument('-v', '--verbose', action='store_true',
                         help='Show verbose output')
     parser.add_argument('--llc-binary', dest='llc', default='llc', type=LLC,