[X86][BtVer2] Teach how to identify zero-idiom VPERM2F128rr instructions.

This patch adds another variant class to identify zero-idiom VPERM2F128rr
instructions.

On Jaguar, a VPERM wih bit 3 and 7 of the mask set, is a zero-idiom.

Differential Revision: https://reviews.llvm.org/D52663

llvm-svn: 343452