When lowering br_table to branches, delete the old jump tables.

This prevents codegen from emitting unused readonly data.

cranelift/filetests/legalizer/br_table_cond.clif

@@ -0,0 +1,40 @@
+test legalizer
+set probestack_enabled=false
+set jump_tables_enabled=false
+target x86_64
+
+; Test that when jump_tables_enables is false, all jump tables are eliminated.
+
+function u0:0(i64 vmctx) baldrdash {
+    gv0 = vmctx
+    gv1 = iadd_imm.i64 gv0, 48
+    jt0 = jump_table [ebb2, ebb2, ebb7]
+    jt1 = jump_table [ebb8, ebb8]
+
+ebb0(v0: i64):
+    jump ebb5
+
+ebb5:
+    v1 = global_value.i64 gv1
+    v2 = load.i64 v1
+    trapnz v2, interrupt
+    v3 = iconst.i32 0
+    br_table v3, ebb3, jt0
+
+ebb7:
+    v4 = iconst.i32 0
+    br_table v4, ebb3, jt1
+
+ebb8:
+    jump ebb5
+
+ebb3:
+    jump ebb2
+
+ebb2:
+    jump ebb1
+
+ebb1:
+    fallthrough_return
+}
+; not: jump_table

lib/codegen/src/legalizer/mod.rs

@@ -110,6 +110,11 @@ pub fn legalize_function(func: &mut ir::Function, cfg: &mut ControlFlowGraph, is
             }
         }
     }
+
+    // Now that we've lowered all br_tables, we don't need the jump tables anymore.
+    if !isa.flags().jump_tables_enabled() {
+        pos.func.jump_tables.clear();
+    }
 }
 
 // Include legalization patterns that were generated by `gen_legalizer.py` from the `XForms` in