[mips][mips64r6] cl[oz], and dcl[oz] are re-encoded in MIPS32r6/MIPS64r6

Summary:
There is no change to the restrictions, just the result register is stored
once in the encoding rather than twice. The rt field is zero in
MIPS32r6/MIPS64r6.

Depends on D4119

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4120