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/filetests/filetests/egraph/isplit.clif

@@ -0,0 +1,18 @@
+test interpret
+test run
+set opt_level=speed_and_size
+set use_egraphs=true
+set enable_llvm_abi_extensions=true
+target x86_64
+target aarch64
+target s390x
+
+function %a(i128) -> i32 {
+block0(v0: i128):
+  v1 = iconst.i32 -1
+  v2, v3 = isplit v0
+  v4 = ushr v1, v3
+  return v4
+}
+
+; run: %a(871558149430564685057836279141) == 2147483647