ZJIT: Add fail-fast assert for non-register {cpush,cpop}_pair

There is no splitting for these so let's add a assert to try and catch
misuse. VRegs are not necessarily registers in the end, so this is best
effort. In those situations they'll get a less proximate panic message.

Author: XrXr

zjit/src/backend/lir.rs

@@ -2159,15 +2159,21 @@ impl Assembler {
         self.push_insn(Insn::CPopInto(opnd));
     }
 
+    #[track_caller]
     pub fn cpop_pair_into(&mut self, opnd0: Opnd, opnd1: Opnd) {
+        assert!(matches!(opnd0, Opnd::Reg(_) | Opnd::VReg{ .. }), "Destination of cpop_pair_into must be a register, got: {opnd0:?}");
+        assert!(matches!(opnd1, Opnd::Reg(_) | Opnd::VReg{ .. }), "Destination of cpop_pair_into must be a register, got: {opnd1:?}");
         self.push_insn(Insn::CPopPairInto(opnd0, opnd1));
     }
 
     pub fn cpush(&mut self, opnd: Opnd) {
         self.push_insn(Insn::CPush(opnd));
     }
 
+    #[track_caller]
     pub fn cpush_pair(&mut self, opnd0: Opnd, opnd1: Opnd) {
+        assert!(matches!(opnd0, Opnd::Reg(_) | Opnd::VReg{ .. }), "Destination of cpush_pair must be a register, got: {opnd0:?}");
+        assert!(matches!(opnd1, Opnd::Reg(_) | Opnd::VReg{ .. }), "Destination of cpush_pair must be a register, got: {opnd1:?}");
         self.push_insn(Insn::CPushPair(opnd0, opnd1));
     }