Change CLIF shuffle to validate lane indices (#5995)

* Change CLIF `shuffle` to validate lane indices

Previously the CLIF `shuffle` instruction did not perform any validation
on the lane shuffle mask and specified that out-of-bounds lanes always
returned 0 as the value. This behavior though is not required by
WebAssembly which validates that lane indices are always in-bounds.
Additionally since these are static immediates even other code
generators should be able to verify that the immediates are in-bounds.

As a result this commit updates the definition of the `shuffle`
instruction to specify that all byte immediates must be in-bounds in the
range of [0, 32). The verifier has been updated and some test cases have
been removed that were testing this functionality.

Closes #5989

* Only generate valid shuffle immediates in fuzzer

cranelift/fuzzgen/src/function_generator.rs

@@ -339,7 +339,10 @@ fn insert_shuffle(
     let rhs = builder.use_var(fgen.get_variable_of_type(ctrl_type)?);
 
     let mask = {
-        let lanes = fgen.u.arbitrary::<[u8; 16]>()?;
+        let mut lanes = [0u8; 16];
+        for lane in lanes.iter_mut() {
+            *lane = fgen.u.int_in_range(0..=31)?;
+        }
         let lanes = ConstantData::from(lanes.as_ref());
         builder.func.dfg.immediates.push(lanes)
     };