Legalize ireduce.iN.i2N to isplit.

cranelift/filetests/filetests/isa/x86/legalize-ireduce-i128.clif

@@ -0,0 +1,11 @@
+test compile
+target x86_64
+
+function u0:0(i64, i64) -> i64 {
+block0(v0: i64, v1: i64):
+    v2 = iconcat v0, v1
+    v3 = ireduce.i64 v2
+    ; check: v3 = copy v0
+    ; check: return v3
+    return v3
+}

cranelift/filetests/filetests/isa/x86/legalize-ireduce-i64.clif

@@ -0,0 +1,11 @@
+test compile
+target i686
+
+function u0:0(i32, i32) -> i32 {
+block0(v0: i32, v1: i32):
+    v2 = iconcat v0, v1
+    v3 = ireduce.i32 v2
+    ; check: v3 = fill v0
+    ; check: return v3
+    return v3
+}