Add back the ifcmp_sp CLIF opcode.

This opcode was removed as part of the old-backend cleanup in #3446.
While this opcode will definitely go away eventually, it is
unfortunately still used today in Lucet (as we just discovered while
working to upgrade Lucet's pinned Cranelift version). Lucet is
deprecated and slated to eventually be completely sunset in favor of
Wasmtime; but until that happens, we need to keep this opcode.

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -3744,7 +3744,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             });
         }
 
-        Opcode::ConstAddr | Opcode::Vconcat | Opcode::Vsplit => {
+        Opcode::ConstAddr | Opcode::Vconcat | Opcode::Vsplit | Opcode::IfcmpSp => {
             return Err(CodegenError::Unsupported(format!(
                 "Unimplemented lowering: {}",
                 op

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

@@ -2888,6 +2888,10 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
 
         Opcode::Isplit | Opcode::Iconcat => unimplemented!("Wide integer ops not implemented."),
 
+        Opcode::IfcmpSp => {
+            panic!("Unused opcode should not be encountered.");
+        }
+
         Opcode::Ifcmp
         | Opcode::Ffcmp
         | Opcode::Trapff

cranelift/codegen/src/isa/x64/lower.rs

@@ -6860,7 +6860,7 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             panic!("table_addr should have been removed by legalization!");
         }
 
-        Opcode::Copy => {
+        Opcode::IfcmpSp | Opcode::Copy => {
             panic!("Unused opcode should not be encountered.");
         }
 
@@ -7075,6 +7075,23 @@ impl LowerBackend for X64Backend {
                         let cond_code = emit_cmp(ctx, ifcmp, cond_code);
                         let cc = CC::from_intcc(cond_code);
                         ctx.emit(Inst::jmp_cond(cc, taken, not_taken));
+                    } else if let Some(ifcmp_sp) = matches_input(ctx, flag_input, Opcode::IfcmpSp) {
+                        let operand = put_input_in_reg(
+                            ctx,
+                            InsnInput {
+                                insn: ifcmp_sp,
+                                input: 0,
+                            },
+                        );
+                        let ty = ctx.input_ty(ifcmp_sp, 0);
+                        ctx.emit(Inst::cmp_rmi_r(
+                            OperandSize::from_ty(ty),
+                            RegMemImm::reg(regs::rsp()),
+                            operand,
+                        ));
+                        let cond_code = ctx.data(branches[0]).cond_code().unwrap();
+                        let cc = CC::from_intcc(cond_code);
+                        ctx.emit(Inst::jmp_cond(cc, taken, not_taken));
                     } else {
                         // Should be disallowed by flags checks in verifier.
                         unimplemented!("Brif with non-ifcmp input");