make small (<= size_of::<int>()) enums immediate

C-like enums are excluded from this for now, because the code paths
specific to them need to be changed.

    fn foo() -> Option<~int> { Some(~5) }

Before:

    ; Function Attrs: uwtable
    define void @_ZN3foo18hdec6e36682b87eeaf4v0.0E(%"enum.std::option::Option<~int>[#1]"* noalias nocapture sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #0 {
    "function top level":
      %2 = tail call %"enum.std::libc::types::common::c95::c_void[#1]"* @"_ZN2rt11global_heap10malloc_raw17h56c543b77f9b78aY11v0.9$x2dpreE"({ i64, %tydesc*, i8*, i8*, i8 }* undef, i64 8)
      %3 = bitcast %"enum.std::libc::types::common::c95::c_void[#1]"* %2 to i64*
      store i64 5, i64* %3, align 8
      %4 = getelementptr inbounds %"enum.std::option::Option<~int>[#1]"* %0, i64 0, i32 0
      store i64* %3, i64** %4, align 8
      ret void
    }

After:

    ; Function Attrs: uwtable
    define %"enum.std::option::Option<~int>[#1]" @_ZN3foo18h2cbf6557a3143edah4v0.0E({ i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #0 {
    "function top level":
      %1 = tail call %"enum.std::libc::types::common::c95::c_void[#1]"* @"_ZN2rt11global_heap10malloc_raw18hb1e9dd1beab35edau11v0.9$x2dpreE"({ i64, %tydesc*, i8*, i8*, i8 }* undef, i64 8)
      %2 = bitcast %"enum.std::libc::types::common::c95::c_void[#1]"* %1 to i64*
      store i64 5, i64* %2, align 8
      %oldret = insertvalue %"enum.std::option::Option<~int>[#1]" undef, i64* %2, 0
      ret %"enum.std::option::Option<~int>[#1]" %oldret
    }

src/librustc/middle/trans/common.rs

@@ -69,9 +69,13 @@ pub fn type_is_immediate(ccx: &mut CrateContext, ty: ty::t) -> bool {
     if simple {
         return true;
     }
+    // FIXME: #9651: C-like enums should also be immediate
+    if ty::type_is_c_like_enum(ccx.tcx, ty) {
+        return false;
+    }
     match ty::get(ty).sty {
-        // FIXME: #9651: small `ty_struct` and `ty_enum` should also be immediate
-        ty::ty_tup(*) => {
+        // FIXME: #9651: small `ty_struct` should also be immediate
+        ty::ty_enum(*) | ty::ty_tup(*) => {
             let llty = sizing_type_of(ccx, ty);
             llsize_of_alloc(ccx, llty) <= llsize_of_alloc(ccx, ccx.int_type)
         }