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/fuzzgen/src/function_generator.rs

@@ -419,8 +419,7 @@ where
 
     /// Generates a br_table into a random block
     fn generate_br_table(&mut self, builder: &mut FunctionBuilder) -> Result<()> {
-        let _type = *self.u.choose(&[I8, I16, I32, I64][..])?;
-        let var = self.get_variable_of_type(_type)?;
+        let var = self.get_variable_of_type(I32)?; // br_table only supports I32
         let val = builder.use_var(var);
 
         let valid_blocks = self.generate_valid_jumptable_target_blocks();