Cranelift: use regalloc2 constraints on caller side of ABI code. (#4892)

* Cranelift: use regalloc2 constraints on caller side of ABI code.

This PR updates the shared ABI code and backends to use register-operand
constraints rather than explicit pinned-vreg moves for register
arguments and return values.

The s390x backend was not updated, because it has its own implementation
of ABI code. Ideally we could converge back to the code shared by x64
and aarch64 (which didn't exist when s390x ported calls to ISLE, so the
current situation is underestandable, to be clear!). I'll leave this for
future work.

This PR exposed several places where regalloc2 needed to be a bit more
flexible with constraints; it requires regalloc2#74 to be merged and
pulled in.

* Update to regalloc2 0.3.3.

In addition to version bump, this required removing two asserts as
`SpillSlot`s no longer carry their class (so we can't assert that they
have the correct class).

* Review comments.

* Filetest updates.

* Add cargo-vet audit for regalloc2 0.3.2 -> 0.3.3 upgrade.

* Update to regalloc2 0.4.0.

cranelift/codegen/src/machinst/isle.rs

@@ -1208,21 +1208,32 @@ macro_rules! isle_prelude_method_helpers {
                 caller.emit_copy_regs_to_buffer(self.lower_ctx, i, *arg_regs);
             }
             for (i, arg_regs) in arg_regs.iter().enumerate() {
-                for inst in caller.gen_copy_regs_to_arg(self.lower_ctx, i, *arg_regs) {
+                for inst in caller.gen_arg(self.lower_ctx, i, *arg_regs) {
                     self.lower_ctx.emit(inst);
                 }
             }
-            caller.emit_call(self.lower_ctx);
-
+            // Handle retvals prior to emitting call, so the
+            // constraints are on the call instruction; but buffer the
+            // instructions till after the call.
             let mut outputs = InstOutput::new();
+            let mut retval_insts: crate::machinst::abi::SmallInstVec<_> = smallvec::smallvec![];
             for i in 0..num_rets {
                 let ret = self.lower_ctx.sigs()[abi].get_ret(i);
                 let retval_regs = self.abi_arg_slot_regs(&ret).unwrap();
-                for inst in caller.gen_copy_retval_to_regs(self.lower_ctx, i, retval_regs.clone()) {
-                    self.lower_ctx.emit(inst);
-                }
+                retval_insts.extend(
+                    caller
+                        .gen_retval(self.lower_ctx, i, retval_regs.clone())
+                        .into_iter(),
+                );
                 outputs.push(valueregs::non_writable_value_regs(retval_regs));
             }
+
+            caller.emit_call(self.lower_ctx);
+
+            for inst in retval_insts {
+                self.lower_ctx.emit(inst);
+            }
+
             caller.emit_stack_post_adjust(self.lower_ctx);
 
             outputs