OPT_EXPERIMENTAL tracking issue

[Wuerfel21]

At some point some of the currently experimental optimizations should become enabled by default. Thus, tracking issue.

Features currently gated by the experimental flag:

• All of OptimizeBranchCommonOps
  • The oldest feature, but also a pretty complex one.
• Conditionalize short sequences with same branch target #346
  • Seems fairly safe?
• Clean up and refactor ReplaceBack optimization to allow loops #365 (loop support)
  • ???
• Inline pure functions when all args are constant #395

[totalspectrum]

#346 does seem pretty safe, I think we can drop the OPT_EXPERIMENTAL for that at least.