chore: remove IRType.{isStruct,isUnion} (#9219)

These are used by the checker for `.ctor`, but I don't think that that
unboxed types will reuse `.ctor`, whose implementation details are
intimately connected to our runtime representation of objects.

src/Lean/Compiler/IR/Basic.lean

@@ -100,14 +100,6 @@ def isIrrelevant : IRType → Bool
   | irrelevant => true
   | _ => false
 
-def isStruct : IRType → Bool
-  | struct _ _ => true
-  | _ => false
-
-def isUnion : IRType → Bool
-  | union _ _ => true
-  | _ => false
-
 end IRType
 
 /-- Arguments to applications, constructors, etc.

src/Lean/Compiler/IR/Checker.lean

@@ -133,7 +133,7 @@ def checkExpr (ty : IRType) (e : Expr) : M Unit := do
       throw s!"constructor '{c.name}' has too many fields"
     if c.ssize + c.usize * usizeSize > maxCtorScalarsSize then
       throw s!"constructor '{c.name}' has too many scalar fields"
-    if !ty.isStruct && !ty.isUnion && c.isRef then
+    if c.isRef then
       checkObjType ty
       checkArgs ys
   | .reset _ x =>