Pass CodeGenOpt::Less to LLVM at O1 (rather than CodeGenOpt::None). (#…

…37893)

For context, please see
#35086 (comment).
Regarding alignment with clang, please see
https://reviews.llvm.org/D28409
(/#35086 (comment)).

```
Prior to Julia 1.5, Julia passed CodeGenOpt::Aggressive to LLVM at -O1.
As of Julia 1.5, Julia passes CodeGenOpt::None to LLVM at -O1.

This reduction in optimization effort at -O1 improved compilation latency,
but induced appreciable runtime regressions.

This commit makes Julia pass CodeGenOpt::Less to LLVM at -O1,
mitigating the runtime regressions caused by CodeGenOpt::None,
while retaining most of CodeGenOpt::None's latency improvements.

This commit also aligns Julia's CodeGenOpt selection at -O1
with that of clang.
```

Best! :)

src/jitlayers.cpp

@@ -608,7 +608,8 @@ CodeGenOpt::Level CodeGenOptLevelFor(int optlevel)
 #ifdef DISABLE_OPT
     return CodeGenOpt::None;
 #else
-    return optlevel < 2 ? CodeGenOpt::None :
+    return optlevel == 0 ? CodeGenOpt::None :
+        optlevel == 1 ? CodeGenOpt::Less :
         optlevel == 2 ? CodeGenOpt::Default :
         CodeGenOpt::Aggressive;
 #endif