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

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spupyrev 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.
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.

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