[PowerPC] Handle __builtin_xxpermdi the same way as GCC does

The codegen for this builtin was initially implemented to match GCC.
However, due to interest from users GCC changed behaviour to account for the
big endian bias of the instruction and correct it. This patch brings the
handling inline with GCC.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38192

Differential Revision: https://reviews.llvm.org/D49424

llvm-svn: 337449