[Utils][update_mc_test_checks] Generate check lines in alphabetical order. (llvm#164424)

Currently, check lines are generated beginning from those with
the most used prefixes, and then if two or more prefixes cover
the same number of RUN lines, they would come in the order of
those RUN lines.

This means adding RUN lines may change the order of check lines,
even if the set of check prefixes remained the same, which is not
ideal.

Author: kosarev