Replace uses of DwarfExpression::addMachineReg* with addMachineRegExpression

and mark the methods as protected.

Besides reducing the surface area of DwarfExpression, this is in
preparation for an upcoming bugfix in the DwarfExpression
implementation, for which it will be necessary to defer emitting
register operations until the rest of the expression is known.

NFC

llvm-svn: 298309