JIT: some lclvars related cleanup (dotnet/coreclr#19077)

JIT: some lclvars related cleanup (dotnet/coreclr#19077)

Consolidate various compiler globals used when setting local var ref
counts by folding them into the visitor:
* lvaMarkRefsCurBlock
* lvaMarkRefsCurStmt
* lvaMarkRefsWeight

Remove the largely vestigial `lvPrefReg` and associated methods to set
or modify this field. Haven't verified but this is likely a remmant of
the legacy backend.

In the one remaning use (lcl var sorting predicates), swap in `lvIsRegArg`
instead, which gets most of the same cases.

Commit migrated from https://github.com/dotnet/coreclr/commit/456d22753b62b5e46b61fa13bef75847229d30ca