Don't allow reuse_def constraints in the s390x Loop instruction (#5336)

cranelift/codegen/src/isa/s390x/inst/mod.rs

@@ -1068,6 +1068,12 @@ fn s390x_get_operands<F: Fn(VReg) -> VReg>(inst: &Inst, collector: &mut OperandC
             for inst in body.iter() {
                 s390x_get_operands(inst, collector);
             }
+
+            // `reuse_def` constraints can't be permitted in a Loop instruction because the operand
+            // index will always be relative to the Loop instruction, not the individual
+            // instruction in the loop body. However, fixed-nonallocatable registers used with
+            // instructions that would have emitted `reuse_def` constraints are fine.
+            debug_assert!(collector.no_reuse_def());
         }
         &Inst::CondBreak { .. } => {}
         &Inst::VirtualSPOffsetAdj { .. } => {}

cranelift/codegen/src/machinst/reg.rs

@@ -4,7 +4,7 @@
 
 use alloc::{string::String, vec::Vec};
 use core::{fmt::Debug, hash::Hash};
-use regalloc2::{Allocation, MachineEnv, Operand, PReg, PRegSet, VReg};
+use regalloc2::{Allocation, MachineEnv, Operand, OperandConstraint, PReg, PRegSet, VReg};
 
 #[cfg(feature = "enable-serde")]
 use serde::{Deserialize, Serialize};
@@ -329,6 +329,16 @@ impl<'a, F: Fn(VReg) -> VReg> OperandCollector<'a, F> {
         }
     }
 
+    /// Returns true if no reuse_def constraints have been added.
+    pub fn no_reuse_def(&self) -> bool {
+        !self.operands[self.operands_start..]
+            .iter()
+            .any(|operand| match operand.constraint() {
+                OperandConstraint::Reuse(_) => true,
+                _ => false,
+            })
+    }
+
     fn is_allocatable_preg(&self, reg: PReg) -> bool {
         self.allocatable.contains(reg)
     }