Move jump tables to the DataFlowGraph (#5745)

Move the storage for jump tables off of FunctionStencil and onto DataFlowGraph. This change is in service of #5731, making it easier to access the jump table data in the context of helpers like inst_values.

cranelift/codegen/src/flowgraph.rs

@@ -137,7 +137,7 @@ impl ControlFlowGraph {
                 } => {
                     self.add_edge(block, inst, *dest);
 
-                    for dest in func.jump_tables[*jt].iter() {
+                    for dest in func.stencil.dfg.jump_tables[*jt].iter() {
                         self.add_edge(block, inst, *dest);
                     }
                 }