[llvm-exegesis] if lit can't find llvm-exegesis, fail gracefully

currently in can_use_perfcounters() in
llvm/test/tools/llvm-exegesis/lit.local.cfg, we check if we can execute
llvm-exegesis. But we don't check if we did actually find llvm-exegesis. If we
didn't, lit.util.which() will return None, and we try to execute that.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D146964