Currently if llc/opt is missing, we get

  checking for llc-3.7... no
  checking for llc... no
  checking  is version 3.7... ./configure[7417]: --version:  not found
  no
  checking for opt-3.7... no
  checking for opt... no
  checking  is version 3.7... ./configure[7664]: --version:  not found
  no
  checking for llc... no
  checking for opt... no

With this fix, the version is queried iff `llc`/`opt` has been detected
at all, thereby avoiding the disturbing `--version: not found` error
output.

Reviewed By: thomie

Differential Revision: https://phabricator.haskell.org/D1674