[Codegen][tablgen][NFC] Allow meta instruction to be target dependent

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