don't allow ZST in ScalarInt

There are several indications that we should not ZST as a ScalarInt:
- We had two ways to have ZST valtrees, either an empty `Branch` or a `Leaf` with a ZST in it.
  `ValTree::zst()` used the former, but the latter could possibly arise as well.
- Likewise, the interpreter had `Immediate::Uninit` and `Immediate::Scalar(Scalar::ZST)`.
- LLVM codegen already had to special-case ZST ScalarInt.

So instead add new ZST variants to those types that did not have other variants
which could be used for this purpose.

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -84,6 +84,10 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 let llval = bx.scalar_to_backend(x, scalar, bx.immediate_backend_type(layout));
                 OperandValue::Immediate(llval)
             }
+            ConstValue::ZST => {
+                let llval = bx.zst_to_backend(bx.immediate_backend_type(layout));
+                OperandValue::Immediate(llval)
+            }
             ConstValue::Slice { data, start, end } => {
                 let Abi::ScalarPair(a_scalar, _) = layout.abi else {
                     bug!("from_const: invalid ScalarPair layout: {:#?}", layout);