reorganize llc checks script to allow more flexibility; NFCI

 
The goal is to enhance this script to be used with opt and clang:
Group all of the regexes together, so it's easier to see what's going on.
This will make it easier to break main() up into pieces too. 
Also, note that some of the regexes are for x86-specific asm. 

llvm-svn: 264197