[Power9] Exploit vector extract with variable index.

This patch adds the exploitation for new power 9 instructions which extract
variable elements from vectors:
VEXTUBLX
VEXTUBRX
VEXTUHLX
VEXTUHRX
VEXTUWLX
VEXTUWRX

Differential Revision: https://reviews.llvm.org/D34032
Commit on behalf of Zaara Syeda (syzaara@ca.ibm.com)

llvm-svn: 307174