[Power9] Exploit vector integer extend instructions

This patch adds build vector patterns to exploit the vector integer
extend instructions:
vextsb2w - Vector Extend Sign Byte To Word
vextsb2d - Vector Extend Sign Byte To Doubleword
vextsh2w - Vector Extend Sign Halfword To Word
vextsh2d - Vector Extend Sign Halfword To Doubleword
vextsw2d - Vector Extend Sign Word To Doubleword

Differential Revision: https://reviews.llvm.org/D33510

llvm-svn: 304992