[RISCV] Restructure compress-opt-select.ll

Two major changes:
- Remove use of sed preprocessing - this was being used to create two
  versions of each test, and the result is much more readable if we
  just duplicate the tests.
- Use a regex for matching the condition.  An upcoming change causes
  us to reverse the branch direction (which doesn't matter to the
  purpose of these tests at all), so using the regex makes the test
  more stable.

Author: preames