Code review cleanup items and moved some items into LEGACY_BACKEND ifdefs

Code review cleanup items and moved some items into LEGACY_BACKEND ifdefs

From Codereview feedback:
Use BAD_VAR_NUM in a couple places instead of (unsigned)-1
In struct InitVarDscInfo
Renamed hasRetBuf to hasRetBufArg

In struct RegState:
Moved field rsCurRegArmNum into LEGACY_BACKEND ifdef
Removed instance field RegState::rsMaxRegArgNum as it is unnecessary
Reordered the fields from largest to smallest

In struct GenTreeCall node:
Moved field gtCallRegUsedMask into LEGACY_BACKEND ifdef

Changes to genRegArgNext to work correct for X64/UNIX (where it is not currently used)
Document the behavaior when given REG_ARG_LAST