[FastISel][AArch64] Implement the FastLowerArguments hook.

[FastISel][AArch64] Implement the FastLowerArguments hook.

This implements basic argument lowering for AArch64 in FastISel. It only
handles a small subset of the C calling convention. It supports simple
arguments that can be passed in GPR and FPR registers.

This should cover most of the trivial cases without falling back to
SelectionDAG.

This fixes <rdar://problem/17890986>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214846 91177308-0d34-0410-b5e6-96231b3b80d8