aclocal.m4: Fix llc/opt detection code
aclocal.m4: Fix llc/opt detection code

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

GaloisInc/halvm-ghc,snoyberg/ghc,shlevy/ghc,tjakway/ghcjvm,oldmanmike/ghc,tjakway/ghcjvm,mettekou/ghc,sdiehl/ghc,snoyberg/ghc,sdiehl/ghc,mcschroeder/ghc,sgillespie/ghc,shlevy/ghc,vikraman/ghc,nushio3/ghc,shlevy/ghc,vikraman/ghc,mcschroeder/ghc,vikraman/ghc,ezyang/ghc,oldmanmike/ghc,mettekou/ghc,vTurbine/ghc,shlevy/ghc,sgillespie/ghc,nushio3/ghc,shlevy/ghc,sgillespie/ghc,mettekou/ghc,olsner/ghc,oldmanmike/ghc,nushio3/ghc,oldmanmike/ghc,ezyang/ghc,mettekou/ghc,vTurbine/ghc,tjakway/ghcjvm,olsner/ghc,mcschroeder/ghc,oldmanmike/ghc,nushio3/ghc,ezyang/ghc,sgillespie/ghc,olsner/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,gridaphobe/ghc,sdiehl/ghc,mettekou/ghc,vikraman/ghc,tjakway/ghcjvm,mcschroeder/ghc,vTurbine/ghc,sgillespie/ghc,oldmanmike/ghc,shlevy/ghc,mcschroeder/ghc,ezyang/ghc,olsner/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,vikraman/ghc,nushio3/ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,nushio3/ghc,ezyang/ghc,GaloisInc/halvm-ghc,vikraman/ghc,sdiehl/ghc,ezyang/ghc,oldmanmike/ghc,sdiehl/ghc,vTurbine/ghc,sdiehl/ghc,nushio3/ghc,gridaphobe/ghc,gridaphobe/ghc,snoyberg/ghc,shlevy/ghc,vTurbine/ghc,tjakway/ghcjvm,sgillespie/ghc,vikraman/ghc,mcschroeder/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,gridaphobe/ghc,mettekou/ghc,mettekou/ghc,vTurbine/ghc,olsner/ghc,sgillespie/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,gridaphobe/ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,snoyberg/ghc,vTurbine/ghc