[Power9] Disable removing extra swaps on P9.

On power 8 we sometimes insert swaps to deal with the difference between
Little-Endian and Big-Endian. The swap removal pass is supposed to clean up
these swaps. On power 9 we don't need this pass since we do not need to insert
the swaps in the first place.

Commiting on behalf of Stefan Pintilie.
Differential Revision: https://reviews.llvm.org/D34627

llvm-svn: 307185