JIT: refactor edge weight representation

[AndyAyersMS]

Modify how we represent edge weights in the jit; instead of being discrete weight values they should be computed as the product of the source block's weight times its successor likelihood. Also, we should just have one weight value instead of the current min/max weights.

The goal here is to change the maintenance burden from updating weights to updating likelihoods, so profile maintenance is more block centric.

Edge weights are currently only used / computed during PGO. If we adopt an edge-based profiling scheme (as proposed in #46882) then edge weights will initially be the primary source of data.

This also lays the groundwork for profile synthesis.

First bit of this has landed: #81738

category:cq
theme:profile-feedback
skill-level:expert
cost:large

[AndyAyersMS]

See master...AndyAyersMS:EdgeWeights for a preliminary attempt.

[AndyAyersMS]

Would like to find room for this in 6.0 but it may be too disruptive and the gains (aside from much cleaner code in the jit proper are unknown). So moving to future.