Teach the x86 floating point stackifier to handle COPY instructions.

This pass runs before COPY instructions are passed to copyPhysReg, so we simply
translate COPY to the proper pseudo instruction. Note that copyPhysReg does not
handle floating point stack copies.

Once COPY is used everywhere, this can be cleaned up a bit, and most of the
pseudo instructions can be removed.

llvm-svn: 107899