[PowerPC] Fix ICE when truncating some vectors

The legalizer would hit an assertion on PowerPC platform when truncating
a vector whose size is not power of 2.  This patch is to add a check to
prevent vectors with such odd-size elements from being custom lowered.

Reviewed By: Hal Finkel

Differential Revision: https://reviews.llvm.org/D65261

llvm-svn: 368654