[FastISel][AArch64] Custom lower sdiv by power-of-2.

Emit an optimized instruction sequence for sdiv by power-of-2 depending on the
exact flag.

This fixes rdar://problem/18224511.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217986 91177308-0d34-0410-b5e6-96231b3b80d8