[llvm] [CodeLayout] Fix X1_Y_X2 and Y_X2_X1 testing for jumps from Y (PR #66592)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> wow that's indeed a bug, thanks for catching it! I'm running some internal tests to see if there is a measurable improvement.

Thanks!

> I wouldn't touch split-threshold, as it doesn't impact build time too much and there are fairly straightforward ways of speeding up the implementation w/o sacrificing the quality.

I am a bit concerned of the time complexity due to the large `ChainSplitThreshold` (128). If `ChainSplitThreshold` doesn't affect compile time that much, keeping the current value (128) should be fine. I just read through the code and haven't tried it yet.

https://github.com/llvm/llvm-project/pull/66592