[PowerPC] Emit VMX loads/stores for aligned ops to avoid adding swaps on LE

Fixes PR30730.
This is a re-commit of a pulled commit. The commit was pulled because some
software projects contained uses of Altivec vectors that violated alignment
requirements. Known issues have now been fixed.

Committing on behalf of Lei Huang.

Differential Revision: https://reviews.llvm.org/D26861

llvm-svn: 301892