Add legalization for popcnt.i128 via narrowing, fixes #1116

cranelift/filetests/filetests/legalizer/popcnt-i128.clif

@@ -0,0 +1,31 @@
+test legalizer
+target i686
+
+function %foo() -> i128 {
+ebb0:
+    v1 = iconst.i64 0x6400000042
+    v2 = iconst.i64 0x7F10100042
+    v3 = iconcat v1, v2
+    v4 = popcnt.i128 v3
+    return v4
+}
+
+; check: v5 = iconst.i32 66
+; check: v6 = iconst.i32 100
+; check: v1 = iconcat v5, v6
+; check: v7 = iconst.i32 0x1010_0042
+; check: v8 = iconst.i32 127
+; check: v2 = iconcat v7, v8
+; check: v3 = iconcat v1, v2
+; check: v9 = popcnt v1
+; check: v10 = popcnt v2
+; check: v12, v13 = isplit v9
+; check: v14, v15 = isplit v10
+; check: v16, v17 = iadd_ifcout v12, v14
+; check: v18 = iadd_ifcin v13, v15, v17
+; check: v11 = iconcat v16, v18
+; check: v4 = uextend.i128 v11
+; check: v19, v20 = isplit v4
+; check: v21, v22 = isplit v19
+; check: v23, v24 = isplit v20
+; check: return v21, v22, v23, v24