Do not transmute immediates to non-immediates.

Author: cjgillot

compiler/rustc_mir_transform/src/gvn.rs

@@ -429,6 +429,16 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 CastKind::Transmute => {
                     let value = self.evaluated[value].as_ref()?;
                     let to = self.ecx.layout_of(to).ok()?;
+                    // `offset` for immediates only supports scalar/scalar-pair ABIs,
+                    // so bail out if the target is not one.
+                    if value.as_mplace_or_imm().is_right() {
+                        match to.abi {
+                            Abi::Scalar(..) | Abi::ScalarPair(..) => {}
+                            _ if to.is_zst() => {}
+                            Abi::Aggregate { .. } if to.fields.count() == 0 => {}
+                            _ => return None,
+                        }
+                    }
                     value.offset(Size::ZERO, to, &self.ecx).ok()?
                 }
                 _ => return None,