[FastISel][AArch64] Add support for fabs intrinsic.

Lower the llvm.fabs intrinsic to the 'fabs' MI instruction.

This fixes rdar://problem/18946552.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221729 91177308-0d34-0410-b5e6-96231b3b80d8