Bug: gather/scatter accepts same registers in destination/src and index

[Michael Zolotukhin]

Hi HJ, Jan,

>> Please open a bug report for each issue if we will fix them
>> separately.
I've opened bugs [1], [2], and [3] for the reported problems. There
are more testcases for Disp8 problem in the Jan's mail, but I think it
could have the same root cause and could be fixed altogether. If not,
I'll file another bugreport.

Thus, the current patch solves PR16489. Is it ok for trunk?

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=16488
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=16489
[3] https://sourceware.org/bugzilla/show_bug.cgi?id=16490

Thanks,
Michael