Only emit ISLE/egraph terms for single-value insts (#5848)

For instructions with no results (such as branches and stores) or
instructions with multiple results (such as add with carry), we have
assertions checking that an optimization rule doesn't try to match on
or construct such instructions.

When we generate terms for matching or constructing instructions, the
terms for these instructions are guaranteed to panic if they're ever
used. So let's just not generate them.

In the future we may wish to generate terms with different types for
these instructions, to make them usable in ISLE rules for optimization
that fall outside our current egraph constraints.

cranelift/codegen/meta/src/gen_inst.rs

@@ -1405,7 +1405,9 @@ fn gen_common_isle(
         IsleTarget::Opt => "Value",
     };
     for inst in instructions {
-        if isle_target == IsleTarget::Opt && inst.format.has_value_list {
+        if isle_target == IsleTarget::Opt
+            && (inst.format.has_value_list || inst.value_results.len() != 1)
+        {
             continue;
         }