Fix lone isplit, when the corresponding iconcat will be created later during legalization

cranelift/filetests/filetests/isa/x86/legalize-isplit-backwards.clif

@@ -0,0 +1,24 @@
+test compile
+target x86_64
+
+function u0:0(i128) -> i64, i64 fast {
+; check: ebb0(v4: i64 [%rdi], v5: i64 [%rsi], v8: i64 [%rbp]):
+ebb0(v0: i128):
+    jump ebb2
+
+ebb1:
+    ; When this `isplit` is legalized, the bnot below is not yet legalized,
+    ; so there isn't a corresponding `iconcat` yet. We should try legalization
+    ; for this `isplit` again once all instrucions have been legalized.
+    v2, v3 = isplit.i128 v1
+    ; return v6, v7
+    return v2, v3
+
+ebb2:
+    ; check: v6 = bnot.i64 v4
+    ; check: v2 -> v6
+    ; check: v7 = bnot.i64 v5
+    ; check: v3 -> v7
+    v1 = bnot.i128 v0
+    jump ebb1
+}