opt: Adding -O0 to opt tool

[GorNishanov]

Summary

Having -O0 in opt allows testing that -O0 optimization
pipeline is built correctly.

[majnemer]

LGTM

test/Transforms/Coroutines/smoketest.ll

 ; Test that all coroutine passes run in the correct order at all optimization
 ; levels and -disable-coroutines removes coroutine passes from the pipeline.
 ;
+; RUN: opt < %s -disable-output -enable-coroutines -debug-pass=Arguments -O0 2>&1 | FileCheck %s
 ; RUN: opt < %s -disable-output -enable-coroutines -debug-pass=Arguments -O1 2>&1 | FileCheck %s
 ; RUN: opt < %s -disable-output -enable-coroutines -debug-pass=Arguments -O2 2>&1 | FileCheck %s
 ; RUN: opt < %s -disable-output -enable-coroutines -debug-pass=Arguments -O3 2>&1 | FileCheck %s
 ; RUN: opt < %s -disable-output -enable-coroutines -debug-pass=Arguments \
 ; RUN:     -coro-early -coro-split -coro-elide -coro-cleanup 2>&1 | FileCheck %s
 ; RUN: opt < %s -disable-output -debug-pass=Arguments 2>&1 \
 ; RUN:     | FileCheck %s -check-prefix=NOCORO
 

tools/opt/opt.cpp

 DisableOptimizations("disable-opt",
                      cl::desc("Do not run any optimization passes"));
 
 static cl::opt<bool>
 StandardLinkOpts("std-link-opts",
                  cl::desc("Include the standard link time optimizations"));
 
 static cl::opt<bool>
+OptLevelO0("O0",
+  cl::desc("Optimization level 0. Similar to clang -O0"));
+
+static cl::opt<bool>
 OptLevelO1("O1",
            cl::desc("Optimization level 1. Similar to clang -O1"));
 
 static cl::opt<bool>
 OptLevelO2("O2",
            cl::desc("Optimization level 2. Similar to clang -O2"));
 
 static cl::opt<bool>
if (DL.isDefault() && !DefaultDataLayout.empty()) {
     M->setDataLayout(DefaultDataLayout);
   }
 
   // Add internal analysis passes from the target machine.
   Passes.add(createTargetTransformInfoWrapperPass(TM ? TM->getTargetIRAnalysis()
                                                      : TargetIRAnalysis()));
 
   std::unique_ptr<legacy::FunctionPassManager> FPasses;
-  if (OptLevelO1 || OptLevelO2 || OptLevelOs || OptLevelOz || OptLevelO3) {
+  if (OptLevelO0 || OptLevelO1 || OptLevelO2 || OptLevelOs || OptLevelOz ||
+      OptLevelO3) {
     FPasses.reset(new legacy::FunctionPassManager(M.get()));
     FPasses->add(createTargetTransformInfoWrapperPass(
         TM ? TM->getTargetIRAnalysis() : TargetIRAnalysis()));
   }
 
   if (PrintBreakpoints) {
     // Default to standard output.
     if (!Out) {
#endif
   // Create a new optimization pass for each one specified on the command line
   for (unsigned i = 0; i < PassList.size(); ++i) {
     if (StandardLinkOpts &&
         StandardLinkOpts.getPosition() < PassList.getPosition(i)) {
       AddStandardLinkPasses(Passes);
       StandardLinkOpts = false;
     }
 
+    if (OptLevelO0 && OptLevelO0.getPosition() < PassList.getPosition(i)) {
+      AddOptimizationPasses(Passes, *FPasses, TM.get(), 0, 0);
+      OptLevelO0 = false;
+    }
+
     if (OptLevelO1 && OptLevelO1.getPosition() < PassList.getPosition(i)) {
       AddOptimizationPasses(Passes, *FPasses, TM.get(), 1, 0);
       OptLevelO1 = false;
     }
 
     if (OptLevelO2 && OptLevelO2.getPosition() < PassList.getPosition(i)) {
       AddOptimizationPasses(Passes, *FPasses, TM.get(), 2, 0);
       OptLevelO2 = false;
if (PrintEachXForm)
           createPrintModulePass(errs(), "", PreserveAssemblyUseListOrder));
   }
 
   if (StandardLinkOpts) {
     AddStandardLinkPasses(Passes);
     StandardLinkOpts = false;
   }
 
+  if (OptLevelO0)
+    AddOptimizationPasses(Passes, *FPasses, TM.get(), 0, 0);
+
   if (OptLevelO1)
     AddOptimizationPasses(Passes, *FPasses, TM.get(), 1, 0);
 
   if (OptLevelO2)
     AddOptimizationPasses(Passes, *FPasses, TM.get(), 2, 0);
 
   if (OptLevelOs)
     AddOptimizationPasses(Passes, *FPasses, TM.get(), 2, 1);