Refactor AArch64 ABI support to extract common bits for shared impl with x64.

We have observed that the ABI implementations for AArch64 and x64 are
very similar; in fact, x64's implementation started as a modified copy
of AArch64's implementation. This is an artifact of both a similar ABI
(both machines pass args and return values in registers first, then the
stack, and both machines give considerable freedom with stack-frame
layout) and a too-low-level ABI abstraction in the existing design. For
machines that fit the mainstream or most common ABI-design idioms, we
should be able to do much better.

This commit factors AArch64 into machine-specific and
machine-independent parts, but does not yet modify x64; that will come
next.

This should be completely neutral with respect to compile time and
generated code performance.

cranelift/codegen/src/machinst/helpers.rs

@@ -0,0 +1,18 @@
+//! Miscellaneous helpers for machine backends.
+
+use crate::ir::Type;
+
+/// Returns the size (in bits) of a given type.
+pub fn ty_bits(ty: Type) -> usize {
+    usize::from(ty.bits())
+}
+
+/// Is the type represented by an integer (not float) at the machine level?
+pub(crate) fn ty_has_int_representation(ty: Type) -> bool {
+    ty.is_int() || ty.is_bool() || ty.is_ref()
+}
+
+/// Is the type represented by a float or vector value at the machine level?
+pub(crate) fn ty_has_float_or_vec_representation(ty: Type) -> bool {
+    ty.is_vector() || ty.is_float()
+}