[PowerPC] Enable interleaved-access vectorization

[PowerPC] Enable interleaved-access vectorization

This adds a basic cost model for interleaved-access vectorization (and a better
default for shuffles), and enables interleaved-access vectorization by default.
The relevant difference from the default cost model for interleaved-access
vectorization, is that on PPC, the shuffles that end up being used are *much*
cheaper than modeling the process with insert/extract pairs (which are
quite expensive, especially on older cores).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246824 91177308-0d34-0410-b5e6-96231b3b80d8