Avoid false abstraction, like get_x_addr. Avoid recomputing data
about the type being manipulated. Use NEON insns for HFA manipulation.
Note that some of the inline assembly will go away in a subsequent patch.
The iOS abi doesn't require padding between arguments, but
that's not what AARCH64_STACK_ALIGN meant. The hardware will
in fact trap if the SP register is not 16 byte aligned.
The set of functions get_homogeneous_type, element_count, and is_hfa
are all intertwined and recompute data. Return a compound quantity
from is_hfa that contains all the data and avoids the recomputation.
Richard Henderson