Fix AArch64 ABI to respect half-caller-save, half-callee-save vec regs.

This PR updates the AArch64 ABI implementation so that it (i) properly
respects that v8-v15 inclusive have callee-save lower halves, and
caller-save upper halves, by conservatively approximating (to full
registers) in the appropriate directions when generating prologue
caller-saves and when informing the regalloc of clobbered regs across
callsites.

In order to prevent saving all of these vector registers in the prologue
of every non-leaf function due to the above approximation, this also
makes use of a new regalloc.rs feature to exclude call instructions'
writes from the clobber set returned by register allocation. This is
safe whenever the caller and callee have the same ABI (because anything
the callee could clobber, the caller is allowed to clobber as well
without saving it in the prologue).

Fixes #2254.

cranelift/codegen/src/machinst/mod.rs

@@ -125,8 +125,6 @@ pub mod abi;
 pub use abi::*;
 pub mod abi_impl;
 pub use abi_impl::*;
-pub mod pretty_print;
-pub use pretty_print::*;
 pub mod buffer;
 pub use buffer::*;
 pub mod adapter;
@@ -156,6 +154,11 @@ pub trait MachInst: Clone + Debug {
     /// Returns true if the instruction is an epilogue placeholder.
     fn is_epilogue_placeholder(&self) -> bool;
 
+    /// Should this instruction be included in the clobber-set?
+    fn is_included_in_clobbers(&self) -> bool {
+        true
+    }
+
     /// Generate a move.
     fn gen_move(to_reg: Writable<Reg>, from_reg: Reg, ty: Type) -> Self;