[MC] Always encode instruction into SmallVector

All users of MCCodeEmitter::encodeInstruction use a raw_svector_ostream
to encode the instruction into a SmallVector. The raw_ostream however
incurs some overhead for the actual encoding.

This change allows an MCCodeEmitter to directly emit an instruction into
a SmallVector without using a raw_ostream and therefore allow for
performance improvments in encoding. A default path that uses existing
raw_ostream implementations is provided.

Reviewed By: MaskRay, Amir

Differential Revision: https://reviews.llvm.org/D145791