[mips][mips64r6] Correct cond names in the cmp.cond.[ds] instructions

Summary:
It seems we accidentally read the wrong column of the table MIPS64r6 spec
and used the names for c.cond.fmt instead of cmp.cond.fmt.

Differential Revision: http://reviews.llvm.org/D4387

llvm-svn: 212607