[TableGen] Emit OperandType enums for RegisterOperands/RegisterClasses

https://reviews.llvm.org/D66773

The OpTypes::OperandType was creating an enum for all records that
inherit from Operand, but in reality there are operands for instructions
that inherit from other types too. In particular, RegisterOperand and
RegisterClass. This commit adds those types to the list of operand types
that are tracked by the OperandType enum.

Patch by: nlguillemot

llvm-svn: 372641