AArch64: simplify calling conventions slightly.

AArch64: simplify calling conventions slightly.

We can eliminate the custom C++ code in favour of some TableGen to
check the same things. Functionality should be identical, except for a
buffer overrun that was present in the C++ code and meant webkit
failed if any small argument needed to be passed on the stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209636 91177308-0d34-0410-b5e6-96231b3b80d8