Split the lvIsMultiRegArgOrRet into two bits

Split the lvIsMultiRegArgOrRet into two bits

Currently for all targets that have MultiReg return LclVars we require that they not be struct promoted.
But on ARM64 we do allow for MultiReg #argument LclVars to be struct promoted.
Thus we have to disentagle this value into two distinct bool values to implement MultiReg returns for Arm64.

Commit migrated from https://github.com/dotnet/coreclr/commit/1472efcd7d07a5238a8ed091a628df843d7ad437