[split] Prevent double legalization of isplit and vsplit

cranelift/filetests/filetests/isa/x86/isplit-not-legalized-twice.clif

@@ -0,0 +1,20 @@
+test compile
+target x86_64
+
+function u0:0(i64, i64) -> i128 system_v {
+ebb0(v0: i64, v1: i64):
+    trap user0
+
+ebb30:
+    v245 = iconst.i64 0
+    v246 = iconcat v245, v245
+    ; The next instruction used to be legalized twice, causing a panic the second time.
+    v250, v251 = isplit.i128 v370
+    v252, v253 = isplit v246
+    trap user0
+
+ebb45:
+    v369 = iconst.i64 0
+    v370 = load.i128 v369
+    trap user0
+}