[llvm] [TailDuplicator] Do not restrict the computed gotos (PR #114990)

[Alexander Kornienko via llvm-commits]

alexfh wrote:

We've found another problem after this commit: the compile time for a number of translation units (protobuf generated, which means a lot of large switch statements) grew from under 5 seconds to "I have yet to see it finish" (it's been running for more than an hour at this point). And the issue seems to be rather widespread in our codebase. So even if the actual problem is elsewhere (which may well be the case, given the analysis here), this commit unfortunately is triggering it without any good workaround AFAIU.

I'm working on a shareable reproducer now, but it looks like we need a revert or a workaround (different than disabling optimization completely, which is not an option) soon. It could be in the shape of keeping the old behavior under a command line option, for example.

If it helps, stack traces captured during the never-ending compilation look mostly like this:
```
  * frame #0: 0x000055555c32fa0c clang`hasSameSuccessors(llvm::MachineBasicBlock&, llvm::SmallPtrSetImpl<llvm::MachineBasicBlock const*>&) [inlined] llvm::SmallPtrSetImplBase::contains_imp(this=0x00007fffffff3710, Ptr=<unavailable>) const at SmallPtrSet.h:230:24
    frame #1: 0x000055555c32f9d3 clang`hasSameSuccessors(llvm::MachineBasicBlock&, llvm::SmallPtrSetImpl<llvm::MachineBasicBlock const*>&) [inlined] llvm::SmallPtrSetImpl<llvm::MachineBasicBlock const*>::count(this=0x00007fffffff3710, Ptr=<unavailable>) const at SmallPtrSet.h:453:12
    frame #2: 0x000055555c32f9d3 clang`hasSameSuccessors(BB=0x0000505fbb0b3a20, Successors=0x00007fffffff3710) at MachineBlockPlacement.cpp:827:21
    frame #3: 0x000055555c32e70a clang`(anonymous namespace)::MachineBlockPlacement::canTailDuplicateUnplacedPreds(this=0x00007fffffff3f80, BB=0x0000505fbc019120, Succ=0x0000505fbcca0fc0, Chain=0x0000505f8b25e000, BlockFilter=<unavailable>) at MachineBlockPlacement.cpp:1220:36
    frame #4: 0x000055555c32cbb1 clang`(anonymous namespace)::MachineBlockPlacement::buildChain(llvm::MachineBasicBlock const*, (anonymous namespace)::BlockChain&, llvm::SmallSetVector<llvm::MachineBasicBlock const*, 16u>*) [inlined] (anonymous namespace)::MachineBlockPlacement::selectBestSuccessor(this=0x00007fffffff3f80, BB=0x0000505fbc019120, Chain=0x0000505f8b25e000, BlockFilter=0x0000000000000000) at MachineBlockPlacement.cpp:1729:9
    frame #5: 0x000055555c32cb32 clang`(anonymous namespace)::MachineBlockPlacement::buildChain(this=0x00007fffffff3f80, HeadBB=0x0000505fbbf2d5e8, Chain=0x0000505f8b25e000, BlockFilter=0x0000000000000000) at MachineBlockPlacement.cpp:1918:19
    frame #6: 0x000055555c327539 clang`(anonymous namespace)::MachineBlockPlacement::buildCFGChains(this=0x00007fffffff3f80) at MachineBlockPlacement.cpp:2826:3
    frame #7: 0x000055555c324328 clang`(anonymous namespace)::MachineBlockPlacement::run(this=0x00007fffffff3f80, MF=0x0000505fbd572d00) at MachineBlockPlacement.cpp:3606:5
    frame #8: 0x000055555c326ee7 clang`(anonymous namespace)::MachineBlockPlacementLegacy::runOnMachineFunction(this=0x0000505fbe193dc0, MF=0x0000505fbd572d00) at MachineBlockPlacement.cpp:657:10
    frame #9: 0x000055555c377fcc clang`llvm::MachineFunctionPass::runOnFunction(this=0x0000505fbe193dc0, F=0x0000505fbe929ef8) at MachineFunctionPass.cpp:108:10
    frame #10: 0x000055555d818135 clang`llvm::FPPassManager::runOnFunction(this=0x0000505fbfe08380, F=0x0000505fbe929ef8) at LegacyPassManager.cpp:1406:27
    frame #11: 0x000055555d81ef7d clang`llvm::FPPassManager::runOnModule(this=0x0000505fbfe08380, M=<unavailable>) at LegacyPassManager.cpp:1452:16
```


https://github.com/llvm/llvm-project/pull/114990