Restrict the types for isplit and iconcat to match backends (#6070)

* Restrict the types for isplit and iconcat to match backends

* Admit unimplemented bitwidths to isplit/iconcat

* Modify the NarrowInt type instead of shadowing it

* Fix filetest failures

cranelift/filetests/filetests/parser/rewrite.clif

@@ -5,13 +5,13 @@ test cat
 ; Defining numbers.
 function %defs() {
 block100(v20: i32):
-    v1000 = iconst.i32x8 5
+    v1000 = iconst.i32 5
     v9200 = f64const 0x4.0p0
     trap user4
 }
 ; sameln: function %defs() fast {
 ; nextln: block100(v20: i32):
-; nextln:     v1000 = iconst.i32x8 5
+; nextln:     v1000 = iconst.i32 5
 ; nextln:     v9200 = f64const 0x1.0000000000000p2
 ; nextln:     trap user4
 ; nextln: }

cranelift/filetests/filetests/parser/tiny.clif

@@ -65,15 +65,13 @@ block0(v95: i32, v96: i32, v97: i8):
 ; nextln: }
 
 ; Lane indexes.
-function %lanes() {
-block0:
-    v0 = iconst.i32x4 2
+function %lanes(i32x4) {
+block0(v0: i32x4):
     v1 = extractlane v0, 3
     v2 = insertlane v0, v1, 1
 }
-; sameln: function %lanes() fast {
-; nextln: block0:
-; nextln:     v0 = iconst.i32x4 2
+; sameln: function %lanes(i32x4) fast {
+; nextln: block0(v0: i32x4):
 ; nextln:     v1 = extractlane v0, 3
 ; nextln:     v2 = insertlane v0, v1, 1
 ; nextln: }