[bugpoint] Don't try to reduce pass list if we only have one

[loladiro]

Summary

If there's only one possible pass, we know that reduction will
do nothing, but currently it still runs a test iterastion on that
one pass. This can take a fair bit of time, particularly since this
runs very early, so the input module is likely to be huge. Just skip
the pass reduction step if there is already only one pass.

[fhahn]

LGTM, thanks!

llvm/tools/bugpoint/CrashDebugger.cpp

 //===- CrashDebugger.cpp - Debug compilation crashes ----------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //

 /// debugOptimizerCrash - This method is called when some pass crashes on input.
 /// It attempts to prune down the testcase to something reasonable, and figure
 /// out exactly which pass is crashing.
 ///
 Error BugDriver::debugOptimizerCrash(const std::string &ID) {
   outs() << "\n*** Debugging optimizer crash!\n";
 
   // Reduce the list of passes which causes the optimizer to crash...
-  if (!BugpointIsInterrupted && !DontReducePassList) {
+  if (!BugpointIsInterrupted && !DontReducePassList && PassesToRun.size() > 1) {
     Expected<bool> Result = ReducePassList(*this).reduceList(PassesToRun);
     if (Error E = Result.takeError())
       return E;
   }
 
   outs() << "\n*** Found crashing pass"
          << (PassesToRun.size() == 1 ? ": " : "es: ")
          << getPassesString(PassesToRun) << '\n';
 
   EmitProgressBitcode(*Program, ID);
 
   auto Res = DebugACrash(*this, TestForOptimizerCrash);
-  if (Res || DontReducePassList)
+  if (Res || DontReducePassList || PassesToRun.size() == 1)
     return Res;
   // Try to reduce the pass list again. This covers additional cases
   // we failed to reduce earlier, because of more complex pass dependencies
   // triggering the crash.
   auto SecondRes = ReducePassList(*this).reduceList(PassesToRun);
   if (Error E = SecondRes.takeError())
     return E;
   outs() << "\n*** Found crashing pass"