Use register numbers instead of copying the full register names. This makes it
possible to change register widths in the middle of a function and keep the
mmreg permutations intact which can be useful for code that only needs larger
vectors for parts of the function in combination with macros etc.

Also change the LOAD_MM_PERMUTATION macro to use the same default name as the
SAVE macro. This simplifies swapping from ymm to xmm registers or vice versa:

    SAVE_MM_PERMUTATION
    INIT_XMM <cpuflags>
    LOAD_MM_PERMUTATION