[mips] Add movep for microMIPS32R6 and fix microMIPS32r3 version

Previously, the 'movep' instruction was defined for microMIPS32r3 and
shared that definition with microMIPS32R6. 'movep' was re-encoded for
microMIPS32r6, so this patch provides the correct encoding.

Secondly, correct the encoding of the 'rs' and 'rt' operands which have
an instruction specific encoding for the registers those operands accept.

Finally, correct the decoding of the 'dst_regs' operand which was extracting
the relevant field from the instruction, but was actually extracting the
field from the alreadly extracted field.

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D39495

llvm-svn: 317475