s390x: Move the value out of the casloop_val_reg with mov_preg (#5430)

The casloop_emit function in the s390x backend was using the fixed non-allocatable register %r0 directly with move instructions, which produced a panic in the regalloc2 checker (#5425). This PR changes the casloop_result function to use mov_preg instead of copy_reg to fetch the result, as it's not viewed by regalloc2 as a move.

Fixes #5425

cranelift/codegen/src/isa/s390x/lower/isle.rs

@@ -938,6 +938,11 @@ impl generated_code::Context for IsleContext<'_, '_, MInst, S390xBackend> {
         stack_reg().to_real_reg().unwrap().into()
     }
 
+    #[inline]
+    fn preg_gpr_0(&mut self) -> PReg {
+        gpr(0).to_real_reg().unwrap().into()
+    }
+
     #[inline]
     fn writable_regpair(&mut self, hi: WritableReg, lo: WritableReg) -> WritableRegPair {
         WritableRegPair { hi, lo }