AArch64: Add support for instruction prefetch intrinsic

Instruction prefetch is not implemented for AArch64, it is incorrectly
translated into data prefetch instruction.

Differential Revision: http://reviews.llvm.org/D4777


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214860 91177308-0d34-0410-b5e6-96231b3b80d8