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/frontend/src/ssa.rs

@@ -601,7 +601,7 @@ impl SSABuilder {
                 let middle_block = dfg.blocks.add();
                 func.stencil.layout.append_block(middle_block);
 
-                let table = &mut func.stencil.jump_tables[*jt];
+                let table = &mut dfg.jump_tables[*jt];
                 for block in table.iter_mut() {
                     if *block == dest_block {
                         *block = middle_block;