IR Outliner Enablement

[lenary]

This is a meta issue for tracking current problems with the IR Outliner - derived from a quick scan of LLVM's issue tracker today. Some of these issues may also affect Hot/Cold Splitting, which is a bit like outlining.

Miscompiles/Crashes:

• Target-specific attributes: [RISCV] Crash when ir-outliner is run on functions with interrupt attribute #149969 (PR: [IROutliner] Add TTI Hook for Propagating Attributes #153985)
• Constants: [IROutliner] Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed #178874 (PR: [IROutliner] Correctly Replace Outlined Constants with Arguments #179885)
• Immargs: main...arsenm:llvm-project:ir-outliner-mishandles-immarg
• Loops? Miscompilation with ir outliner #74648
• Function Specialisation: clang crashes when compiling OpenCV with O3 + function-speciallization + ir-outliner  #54121
• PHI Nodes: IR Outliner can mix up order of incoming values when compressing phi nodes if contain the same values #54211
• @llvm.eh.typeid.for Make @llvm.eh.typeid.for outlining-friendly #38893

Usability:

• Make nooutline accessible from clang: [clang] [outliner] Make nooutline accessible from clang #144135 (PR: [clang] Add clang::no_outline Attribute #163666)
• Ensure outlining flags work with LTO: At least -mno-outline and -mllvm should be redirected somehow to the linker when using LTO #39695 (PR: [clang] Ensure -mno-outline adds attributes #163692)

Compile-time Problems:

• IR Outliner: Huge compile time overhead on MicroBenchmarks/LCALS/LCALSSuite at -O3 #54441
• [IROutliner] CTMark/consumer-typeset slow to compile for AArch64 @ -Oz and -O2 #55393

Cost Model Issues:

• [IROutliner] MultiSource/Benchmarks/MiBench/network-patricia is larger with IR outliner enabled #55395
• [IROutliner] MultiSource/Benchmarks/Prolangs-C++/simul larger with IR Outliner @ -Oz on AArch64 #55397