[WIP][IROutliner] Enable the IR outliner post-link for full LTO

[lewis-revill]

Summary

This patch simply adds the IR outliner to the pipeline of optimizations run under FullLTO post-link. This seems like the right thing to do to capture more candidates, and it provides a small benefit for code size on Embench on riscv32.

Work in progress because I still need to check which tests need to be updated, but submitted for discussion as is.

llvm/lib/Passes/PassBuilderPipelines.cpp

PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   // Run a second time to clean up any type tests left behind by WPD for use
   // in ICP (which is performed earlier than this in the regular LTO pipeline).
   MPM.addPass(LowerTypeTestsPass(nullptr, nullptr, true));
 
   // Enable splitting late in the FullLTO post-link pipeline.
   if (EnableHotColdSplit)
     MPM.addPass(HotColdSplittingPass());
 
+  // Enable IR outlining late in the FullLTO post-link pipeline.
+  if (EnableIROutliner)
+    MPM.addPass(IROutlinerPass());
+
   // Add late LTO optimization passes.
   // Delete basic blocks, which optimization passes may have killed.
   MPM.addPass(createModuleToFunctionPassAdaptor(SimplifyCFGPass(
       SimplifyCFGOptions().convertSwitchRangeToICmp(true).hoistCommonInsts(
           true))));
 
   // Drop bodies of available eternally objects to improve GlobalDCE.
   MPM.addPass(EliminateAvailableExternallyPass());