check or check-next the first line of the function too

We could make this an option if people don't like it. 
But since part of the reason for using a script to generate
checks is to prevent lazy checking that lets bugs crawl 
through, let's have the script check the first line too. 

For asm tests, it ensures that nothing unexpected has 
happened before the first line of asm. This matches the
existing behavior of update_llc_test_checks.py. 

More discussion in PR22897:
https://llvm.org/bugs/show_bug.cgi?id=22897

llvm-svn: 265414