[AVR][NFC] Merge AVRMCCodeEmitter::emitInstruction into AVRMCCodeEmitter::encodeInstruction.

The reasons are:
1, `AVRMCCodeEmitter::emitInstruction` has only one use which is `AVRMCCodeEmitter::encodeInstruction`, and the parameter `STI` is not used in this function. I think it might be copied from other target.
2, We do have `AVRAsmPrinter::emitInstruction`, and it would invoke `AVRMCCodeEmitter::encodeInstruction` in its calling chain, so if we call `AVRMCCodeEmitter::emitInstruction` in `AVRMCCodeEmitter::encodeInstruction`, it would be confusing.

Reviewed By: benshi001

Differential Revision: https://reviews.llvm.org/D155426