[utils] remove ability to generate llc check lines from update_test_checks.py

[utils] remove ability to generate llc check lines from update_test_checks.py

The dream of a unified check-line auto-generator for all phases of compilation is dead.
The llc script has already diverged to be better at its goal, so having 2 scripts that
do almost the same thing just causes confusion. Now, this script will only work with
opt to produce check lines for IR transforms.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305208 91177308-0d34-0410-b5e6-96231b3b80d8