MIPS/opcodes: Remove the regular MIPS "+t" operand code

The semantics of the regular MIPS "+t" operand code is exactly the same
as that of the "E" operand code, so replace the former with the latter
in the single MFTC0 instruction with implicit 'sel' == 0 encoding where
it's used, matching the encoding with explicit 'sel' as well as other
instructions.