Add a distinct OperandValue::ZeroSized variant for ZSTs

These tend to have special handling in a bunch of places anyway, so the variant helps remember that.  And I think it's easier to grok than non-Scalar Aggregates sometimes being `Immediates` (like I got wrong and caused 109992).  As a minor bonus, it means we don't need to generate poison LLVM values for them to pass around in `OperandValue::Immediate`s.

compiler/rustc_codegen_ssa/src/base.rs

@@ -295,7 +295,7 @@ pub fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             let (base, info) = match bx.load_operand(src).val {
                 OperandValue::Pair(base, info) => unsize_ptr(bx, base, src_ty, dst_ty, Some(info)),
                 OperandValue::Immediate(base) => unsize_ptr(bx, base, src_ty, dst_ty, None),
-                OperandValue::Ref(..) => bug!(),
+                OperandValue::Ref(..) | OperandValue::ZeroSized => bug!(),
             };
             OperandValue::Pair(base, info).store(bx, dst);
         }