Streamline LSRA for non-enregistered lclVars

Streamline LSRA for non-enregistered lclVars

- Don't create any lclVar intervals if no lclVars are ever enregistered.
- Don't create intervals for tracked lclVars that are not register candidates.
- Don't create any `RegToVarMap`s when no lclVars are ever enregistered.
- Don't do edge resolution if no lclVars are enregistered.
- Eliminate non-candidate lclVars from the sets used by LSRA.

Also:
- Change VarToRegMap to use regNumberSmall.
- Enhance `COMPLus_JitLsraStats` and `COMPlus_JitTimeLogCsv` dumping.

Commit migrated from https://github.com/dotnet/coreclr/commit/47050655f1ce630ddcdd999bcde9a535d9b3b9dd