[PowerPC] Disable direct moves for extractelement and bitcast in 32-bit mode

This patch corresponds to review:
http://reviews.llvm.org/D17711

It disables direct moves on these operations in 32-bit mode since the patterns
assume 64-bit registers. The final patch is slightly different from the
Phabricator review as the bitcast operations needed to be disabled in 32-bit
mode as well. This fixes PR26617.

llvm-svn: 264282