When transforming a Phi into a CMove, the threshold is set to be approximately BlockLayoutMinDiamondPercentage, the reason is given:

// BlockLayoutByFrequency optimization moves infrequent branch
// from hot path. No point in CMOV'ing in such case

This sets the default value of the threshold to be around 18%, which is too conservative. The reason also does not make a lot of sense since the important property which makes jumping expensive is not code layout. We should remove this.