[UpdateTestChecks] Change shebang from python to python3

[MaskRay]

Summary

'python' means Python 2 on some platforms while Python 3 on others.
'python3' is Python 3 only. Python 2.7 End of Life is set to January 1,

2020. Getting rid of Python 2 support reduces maintenance burden.

[lebedev.ri]

Can do this change for everything llvm/utils/update_*-related at once?

[MaskRay]

Update all update_*_checks.py

[lebedev.ri]

I'm okay with this.

llvm/utils/update_analyze_test_checks.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A script to generate FileCheck statements for 'opt' analysis tests.
 
 This script is a utility to update LLVM opt analysis test cases with new
 FileCheck patterns. It can either update all of the tests in the file or
 a single test function.
 
 Example usage:

llvm/utils/update_llc_test_checks.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A test case update script.
 
 This script is a utility to update LLVM 'llc' based test cases with new
 FileCheck patterns. It can either update all of the tests in the file or
 a single test function.
 """
 
def main():
   args = common.parse_commandline_args(parser)
 
   script_name = os.path.basename(__file__)
 
   test_paths = [test for pattern in args.tests for test in glob.glob(pattern)]
   for test in test_paths:
     with open(test) as f:
       input_lines = [l.rstrip() for l in f]
-    
     first_line = input_lines[0] if input_lines else ""
     if 'autogenerated' in first_line and script_name not in first_line:
       common.warn("Skipping test which wasn't autogenerated by " + script_name, test)
       continue
 
     if args.update_only:
       if not first_line or 'autogenerated' not in first_line:
         common.warn("Skipping test which isn't autogenerated: " + test)

llvm/utils/update_mca_test_checks.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A test case update script.
 
 This script is a utility to update LLVM 'llvm-mca' based test cases with new
 FileCheck patterns.
 """
 
 import argparse

llvm/utils/update_mir_test_checks.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Updates FileCheck checks in MIR tests.
 
 This script is a utility to update MIR based tests with new FileCheck
 patterns.
 
 The checks added by this script will cover the entire body of each
 function it handles. Virtual registers used are given names via

llvm/utils/update_test_checks.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A script to generate FileCheck statements for 'opt' regression tests.
 
 This script is a utility to update LLVM opt test cases with new
 FileCheck patterns. It can either update all of the tests in the file or
 a single test function.
 
 Example usage: