[Bug target/71805] incorrect code for test pr45752.c with -mcpu=power9

[meissner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71805

--- Comment #2 from Michael Meissner <meissner at gcc dot gnu.org> ---
It turns out, when I implemented the xxperm variant (and when Kelvin used my
code to add xxpermr) I mis-read the ISA 3.0 manual.  I thought:

    xxperm a,b,c

was equivalent to:

    vperm a,a,b,c

when in reality, it is equivalent to:

    vperm a,b,a,c

(i.e. the XT field provides the register with bytes 16-31 instead of 0-15).