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/frontend.rs

@@ -139,6 +139,8 @@ impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
                 for dest_block in self
                     .builder
                     .func
+                    .stencil
+                    .dfg
                     .jump_tables
                     .get(*table)
                     .expect("you are referencing an undeclared jump table")