llvm dev - Dec 2020 - Instruction namespace

When I think of a "target opcode," I think of opcodes for actual
target machines. The "TargetOpcode" namespace contains the generic
target-independent opcodes. I have to start thinking of all the opcodes as
target opcodes, and the machine-specific opcodes as machine opcodes or native
opcodes.

Perhaps I would have called it the GenericOpcode namespace.