Fix some egraph-related issues. (#5088)

This fixes #5086 by addressing two separate issues:

- The `ValueDataPacked::set_type()` helper had an embarrassing bitfield-manipulation bug that would mangle the rest of a `ValueDef` when setting its type. This is not normally used, only when the egraph elaboration fills in types after-the-fact on a multi-value node.
- The lowering rules for `isplit` on aarch64 and s390x were dispatching on the first output type, rather than the input type. When only the second output is used (as in the example in #5086), the first output type actually remains `INVALID` (and this is fine because it's never used).

cranelift/codegen/src/ir/dfg.rs

@@ -523,7 +523,7 @@ impl ValueDataPacked {
 
     #[inline(always)]
     fn set_type(&mut self, ty: Type) {
-        self.0 &= !((1 << Self::TYPE_BITS) - 1) << Self::TYPE_SHIFT;
+        self.0 &= !(((1 << Self::TYPE_BITS) - 1) << Self::TYPE_SHIFT);
         self.0 |= (ty.repr() as u64) << Self::TYPE_SHIFT;
     }
 }