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_llvm/src/common.rs

@@ -13,7 +13,6 @@ use rustc_codegen_ssa::traits::*;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::{ConstAllocation, GlobalAlloc, Scalar};
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
-use rustc_middle::ty::ScalarInt;
 use rustc_target::abi::{self, AddressSpace, HasDataLayout, Pointer, Size};
 
 use libc::{c_char, c_uint};
@@ -223,13 +222,13 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         })
     }
 
+    fn zst_to_backend(&self, _llty: &'ll Type) -> &'ll Value {
+        self.const_undef(self.type_ix(0))
+    }
+
     fn scalar_to_backend(&self, cv: Scalar, layout: abi::Scalar, llty: &'ll Type) -> &'ll Value {
         let bitsize = if layout.is_bool() { 1 } else { layout.size(self).bits() };
         match cv {
-            Scalar::Int(ScalarInt::ZST) => {
-                assert_eq!(0, layout.size(self).bytes());
-                self.const_undef(self.type_ix(0))
-            }
             Scalar::Int(int) => {
                 let data = int.assert_bits(layout.size(self));
                 let llval = self.const_uint_big(self.type_ix(bitsize), data);