Enable the ssa verifier in debug builds (#5354)

Enable regalloc2's SSA verifier in debug builds to check for any outstanding reuse of virtual registers in def constraints. As fuzzing enables debug_assertions, this will enable the SSA verifier when fuzzing as well.

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -1171,6 +1171,13 @@ impl MachInst for Inst {
         }
     }
 
+    fn is_trap(&self) -> bool {
+        match self {
+            Self::Udf { .. } => true,
+            _ => false,
+        }
+    }
+
     fn is_args(&self) -> bool {
         match self {
             Self::Args { .. } => true,

cranelift/codegen/src/isa/riscv64/inst/mod.rs

@@ -686,6 +686,13 @@ impl MachInst for Inst {
         }
     }
 
+    fn is_trap(&self) -> bool {
+        match self {
+            Self::Udf { .. } => true,
+            _ => false,
+        }
+    }
+
     fn is_args(&self) -> bool {
         match self {
             Self::Args { .. } => true,

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

@@ -1127,6 +1127,13 @@ impl MachInst for Inst {
         }
     }
 
+    fn is_trap(&self) -> bool {
+        match self {
+            Self::Trap { .. } => true,
+            _ => false,
+        }
+    }
+
     fn is_args(&self) -> bool {
         match self {
             Self::Args { .. } => true,

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -2244,6 +2244,13 @@ impl MachInst for Inst {
         }
     }
 
+    fn is_trap(&self) -> bool {
+        match self {
+            Self::Ud2 { .. } => true,
+            _ => false,
+        }
+    }
+
     fn is_args(&self) -> bool {
         match self {
             Self::Args { .. } => true,