Replace InsertLane format with TernaryImm8

The InsertLane format has an ordering (`value().imm().value()`) and immediate name (`"lane"`) that make it awkward to use for other instructions. This changes the ordering (`value().value().imm()`) and uses the default name (`"imm"`) throughout the codebase.

cranelift/filetests/filetests/parser/tiny.clif

@@ -67,13 +67,13 @@ function %lanes() {
 block0:
     v0 = iconst.i32x4 2
     v1 = extractlane v0, 3
-    v2 = insertlane v0, 1, v1
+    v2 = insertlane v0, v1, 1
 }
 ; sameln: function %lanes() fast {
 ; nextln: block0:
 ; nextln:     v0 = iconst.i32x4 2
 ; nextln:     v1 = extractlane v0, 3
-; nextln:     v2 = insertlane v0, 1, v1
+; nextln:     v2 = insertlane v0, v1, 1
 ; nextln: }
 
 ; Integer condition codes.