[Utils] Do not remove comments in llc test script

When checking if two prefixes can be merged for a function,
update_llc_test_checks.py removed IR comments before comparing
llc outputs of different RUN lines.
This means, if one RUN line emited lines starting with ';' and another
RUN line emited the same lines except the ones starting with ';', both
RUNs would be merged (if they share a prefix).

However, CHECK-NEXT lines check the comments, otherwise they fail, so
the script should not merge RUNs if they contain different comments.

Differential Revision: https://reviews.llvm.org/D101312