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.

src/builder.rs

@@ -758,7 +758,7 @@ fn load_operand(&mut self, place: PlaceRef<'tcx, RValue<'gcc>>) -> OperandRef<'t
         assert_eq!(place.llextra.is_some(), place.layout.is_unsized());
 
         if place.layout.is_zst() {
-            return OperandRef::new_zst(self, place.layout);
+            return OperandRef::zero_sized(place.layout);
         }
 
         fn scalar_load_metadata<'a, 'gcc, 'tcx>(bx: &mut Builder<'a, 'gcc, 'tcx>, load: RValue<'gcc>, scalar: &abi::Scalar) {

src/type_of.rs

@@ -159,8 +159,7 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
     fn is_gcc_immediate(&self) -> bool {
         match self.abi {
             Abi::Scalar(_) | Abi::Vector { .. } => true,
-            Abi::ScalarPair(..) => false,
-            Abi::Uninhabited | Abi::Aggregate { .. } => self.is_zst(),
+            Abi::ScalarPair(..) | Abi::Uninhabited | Abi::Aggregate { .. } => false,
         }
     }