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

[stuij]

Summary

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.

[stuij]

amend commit message

llvm/test/tools/llvm-exegesis/lit.local.cfg

 import subprocess
 import lit.util
 
 
 def can_execute_generated_snippets(arch):
     is_host_arch = arch in config.root.host_triple
     # 'native' feature is defined as "host arch == default triple arch"
     is_native_codegen = 'native' in config.available_features
     return is_host_arch and is_native_codegen
 
 
 def can_use_perf_counters(mode, extra_options=[]):
     # We need libpfm to be installed and allow reading perf counters. We can
     # only know that at runtime, so we try to measure an empty code snippet
     # and bail out on error.
     llvm_exegesis_exe = lit.util.which('llvm-exegesis', config.llvm_tools_dir)
+    if llvm_exegesis_exe is None:
+       print('could not find llvm-exegesis')
+       return False
     try:
         return_code = subprocess.call(
             [llvm_exegesis_exe, '-mode', mode, '-snippets-file', '/dev/null'] + extra_options,
             stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
         return return_code == 0
     except OSError:
         print('could not exec llvm-exegesis')
         return False