JIT: fast path for minopts/debug codegen in lvaMarkRefs (dotnet/coreclr#19103)

JIT: fast path for minopts/debug codegen in lvaMarkRefs (dotnet/coreclr#19103)

For minopts and debug codegen, consider all locals to be implicitly
referenced.

This is set up during `lvaMarkLocalVars` and maintained after that
by having `incRefCnts` set the implicit reference bit and not doing
anything in `decRefCnts` for minopts / debug.

Likewise suppress local var sorting, as we don't have accurate counts
to go by.

Commit migrated from https://github.com/dotnet/coreclr/commit/46b1ebabe1536ece7b808407af1f6d1dc5b1ba73