Use CodegenOpts::less when creating a TargetMachine for clang `-O1`

[mehdi_amini]

Summary

Clang was initializing the TargetMachine with CodeGenOpt::Default
for O1. This change is aligning it on llc:

-O0: OptLevel = CodeGenOpt::None
-O1: OptLevel = CodeGenOpt::Less
-O2 -Os -Oz: OptLevel = CodeGenOpt::Default
-O3: OptLevel = CodeGenOpt::Aggressive

[mehdi_amini]

clang-format

[echristo]

Sounds fine to me. I want to modify O1 at some point, but there's nothing wrong with consistency for now.

clang/lib/CodeGen/BackendUtil.cpp

RM = llvm::StringSwitch<llvm::Reloc::Model>(CodeGenOpts.RelocationModel)
            .Case("static", llvm::Reloc::Static)
            .Case("pic", llvm::Reloc::PIC_)
            .Case("ropi", llvm::Reloc::ROPI)
            .Case("rwpi", llvm::Reloc::RWPI)
            .Case("ropi-rwpi", llvm::Reloc::ROPI_RWPI)
            .Case("dynamic-no-pic", llvm::Reloc::DynamicNoPIC);
   assert(RM.hasValue() && "invalid PIC model!");
 
-  CodeGenOpt::Level OptLevel = CodeGenOpt::Default;
+  CodeGenOpt::Level OptLevel;
   switch (CodeGenOpts.OptimizationLevel) {
-  default: break;
-  case 0: OptLevel = CodeGenOpt::None; break;
-  case 3: OptLevel = CodeGenOpt::Aggressive; break;
+  default:
+    llvm_unreachable("Invalid optimization level!");
+  case 0:
+    OptLevel = CodeGenOpt::None;
+    break;
+  case 1:
+    OptLevel = CodeGenOpt::Less;
+    break;
+  case 2:
+    OptLevel = CodeGenOpt::Default;
+    break; // O2/Os/Oz
+  case 3:
+    OptLevel = CodeGenOpt::Aggressive;
+    break;
   }
 
   llvm::TargetOptions Options;
 
   Options.ThreadModel =
     llvm::StringSwitch<llvm::ThreadModel::Model>(CodeGenOpts.ThreadModel)
       .Case("posix", llvm::ThreadModel::POSIX)
       .Case("single", llvm::ThreadModel::Single);