[PowerPC] Update P9 vector costs for insert/extract element

The PPC vector cost model values for insert/extract element reflect older
processors that lacked vector insert/extract and move-to/move-from VSR
instructions.  Update getVectorInstrCost to give appropriate values for when
the newer instructions are present.

Differential Revision: https://reviews.llvm.org/D60160

llvm-svn: 359313