Fix isplit legalization for ebb params when jumping forward

Fixes #1106

cranelift/filetests/filetests/isa/x86/i128-isplit-forward-jump.clif

@@ -0,0 +1,22 @@
+test compile
+target x86_64
+
+function u0:0() -> i128 system_v {
+ebb0:
+    v0 = iconst.i64 0
+    v1 = iconst.i64 0
+    v2 = iconcat v0, v1
+    jump ebb5
+
+ebb2:
+    jump ebb4(v27)
+
+ebb4(v23: i128):
+    return v23
+
+ebb5:
+    v27 = bxor.i128 v2, v2
+    v32 = iconst.i32 0
+    brz v32, ebb2
+    trap user0
+}