An instruction is a meta-instruction if it doesn't produce any output
in the form of executable instructions. So in the concept, a
meta-instruction does not have to be target independent.
Before this patch, isMetaInstruction is implemented by checking the
opcode of the instruction, add we have no way to add target dependent
opcode to the list, which does not make sense.
After this patch, a bit isMeta is added for class Instruction in
tablegen, which is used to indicate whether it's a meta instruction.
clang-format: please reformat the code