[bugpoint] Report non-existent opt binary

Bugpoint will keep going even if the opt binary it's given doesn't
exist. It should at least alert the user, so it's clear why reductions
are failing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324713 91177308-0d34-0410-b5e6-96231b3b80d8

Author: vedantk

test/BugPoint/unsymbolized.ll

@@ -4,6 +4,7 @@
 ; RUN: echo "exit(1)" >> %t.py
 ; RUN: not bugpoint -load %llvmshlibdir/BugpointPasses%shlibext %s -output-prefix %t -bugpoint-crashcalls -opt-command="%python" -opt-args %t.py | FileCheck %s
 ; RUN: not --crash opt -load %llvmshlibdir/BugpointPasses%shlibext %s -bugpoint-crashcalls -disable-symbolication 2>&1 | FileCheck --check-prefix=CRASH %s
+; RUN: not bugpoint -load %llvmshlibdir/BugpointPasses%shlibext %s -output-prefix %t -bugpoint-crashcalls -opt-command=%t.non.existent.opt.binary -opt-args %t.py 2>&1 | FileCheck %s --check-prefix=BAD-OPT
 
 ; Test that bugpoint disables symbolication on the opt tool to reduce runtime overhead when opt crashes
 ; CHECK: args = {{.*}}'-disable-symbolication'
@@ -15,6 +16,7 @@
 ; now.
 ; CRASH-NOT: Signals.inc
 
+; BAD-OPT: Specified `opt' binary does not exist: {{.*}}non.existent.opt.binary
 define void @f() {
   call void @f()
   ret void

tools/bugpoint/OptimizerDriver.cpp

@@ -177,6 +177,10 @@ bool BugDriver::runPasses(Module *Program,
     errs() << "Cannot find `opt' in PATH!\n";
     return 1;
   }
+  if (!sys::fs::exists(tool)) {
+    errs() << "Specified `opt' binary does not exist: " << tool << "\n";
+    return 1;
+  }
 
   std::string Prog;
   if (UseValgrind) {