JIT: Don't put cold blocks in RPO during layout

[amanasifkhalid]

Split off from #112004. Excluding cold blocks from the loop-aware RPO simplifies how we compute the initial layout to feed into 3-opt, as it eliminates the need to manually move cold blocks out-of-line, instead allowing them to sink to the end of the method. This has the consequence of changing -- most likely worsening -- the layout of cold sections. However, our current threshold for "cold" is low enough that I don't think this churn matters: While BB_COLD_WEIGHT is 0.01, we compare this to normalized weights scaled by BB_UNITY_WEIGHT (100), so normalized weights must be below 0.0001 to be considered cold. In other words, profile data must suggest a block executes less than 0.01% (not 1%) of the time to be excluded from reordering.

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[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. Diffs are missing benchmark collections, but it looks like we're frequently shortening jumps on x64/x86 such that it's a size improvement, whereas the loss of fallthrough in the cold section is increasing code size on arm32/arm64 (though if this code is truly cold, it shouldn't matter). Thanks!

[amanasifkhalid]

ping @dotnet/jit-contrib