cranelift: Restrict br_table to i32 indices (#4510)

* cranelift: Restrict `br_table` to `i32` indices

In #4498 it was proposed that we should only accept `i32` indices
to `br_table`. The rationale for this is that larger types lead the
users to a false sense of flexibility (since we don't support jump
tables larger than u32's), and narrower types are not well tested
paths that would be safer if we removed them.

* cranelift: Reduce directly from i128 to i32 in Switch

cranelift/codegen/meta/src/shared/instructions.rs

@@ -157,7 +157,12 @@ fn define_control_flow(
     }
 
     {
-        let x = &Operand::new("x", iB).with_doc("index into jump table");
+        let _i32 = &TypeVar::new(
+            "i32",
+            "A 32 bit scalar integer type",
+            TypeSetBuilder::new().ints(32..32).build(),
+        );
+        let x = &Operand::new("x", _i32).with_doc("i32 index into jump table");
         let JT = &Operand::new("JT", &entities.jump_table);
 
         ig.push(

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

@@ -3244,19 +3244,13 @@ impl LowerBackend for X64Backend {
                     let jt_size = jt_size as u32;
 
                     let ty = ctx.input_ty(branches[0], 0);
-                    let ext_spec = match ty {
-                        types::I128 => panic!("BrTable unimplemented for I128"),
-                        types::I64 => ExtSpec::ZeroExtendTo64,
-                        _ => ExtSpec::ZeroExtendTo32,
-                    };
-
                     let idx = extend_input_to_reg(
                         ctx,
                         InsnInput {
                             insn: branches[0],
                             input: 0,
                         },
-                        ext_spec,
+                        ExtSpec::ZeroExtendTo32,
                     );
 
                     // Emit the compound instruction that does: