MIPS64: Disassembler support for rotate instructions.

Also, tighten the tests for recognizing the various shift commands. The
tests, previously, would be unable to distinguish between "shift right
logical" and "rotate right" commands. In particular:

- SRLV vs. ROTRV
- DSRLV vs. DROTRV,
- DSRL vs. DROTR, and
- DSRL32 vs. DROTR32

Change-Id: I7a6df8ab0d76fd3d34b1207da9915369ad84fa97