AArch64: allocate small fixed args properly in varargs functions.

On Darwin, function arguments occupy their real size when passed on the stack
(e.g. an i16 only consumes 2 bytes). This means that, even for fixed args in
varargs calls we need to keep track of the original type being passed before
any DAG/GISel promotions. Existing logic only applied this fix to the
non-varargs case leading to mismatch between caller & callee in those
situations.

On Linux & Windows these arguments always occupy a 64-bit slot anyway so
there's no special handling needed.