[AArch64] Prefer DUP/MOV ("CPY") to INS for vector_extract.

[AArch64] Prefer DUP/MOV ("CPY") to INS for vector_extract.

This avoids a partial false dependency on the previous content of
the upper lanes of the destination vector register.

Differential Revision: http://reviews.llvm.org/D7307

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227820 91177308-0d34-0410-b5e6-96231b3b80d8