Target: Allow target specific operand types

This adds two new fields to the RegisterOperand TableGen class:

string OperandNamespace = "MCOI";
string OperandType = "OPERAND_REGISTER";

These fields can be used to specify a target specific operand type,
which will be stored in the OperandType member of the MCOperandInfo
object.

This can be useful for targets that need to store some extra information
about operands that cannot be expressed using the target independent
types.  For example, in the R600 backend, there are operands which
can take either registers or immediates and it is convenient to be able
to specify this in the TableGen definitions.

llvm-svn: 225661