Expose move to/from coprocessor instructions in MIPS64 mode.

Note: [D]M{T,F}CP2 is just a recommended encoding.  Vendors often provide a
custom CP2 that interprets instructions differently and may wish to add their
own instructions that use this opcode.  We should ensure that this is easy to
do.  I will probably add a 'has custom CP{0-3}' subtarget flag to make this
easy: We want to avoid the GCC situation where every MIPS vendor makes a custom
fork that breaks every other MIPS CPU and so can't be merged upstream.

llvm-svn: 165711