The implementation follows the MIPS backend and expands the pseudo instruction directly during asm parsing. As the result, only real MC instructions are emitted to the MCStreamer. The actual expansion to real instructions is similar to the expansion performed by the GNU Assembler.
This patch supersedes D41949.
Note that I marked this patch as WIP because I am currently traveling but plan to add compression tests to this patch in an update next week. Comments on the current state are appreciated though.