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.
Reviewed By: pengfei
Differential Revision: https://reviews.llvm.org/D121600
9e832a67fe