Use zero_mem instead of a zerointializer for init intrinsic

LLVM gets overwhelmed when presented with a zeroinitializer for a large
type. In unoptimised builds, it generates a long sequence of stores to
memory. In optmised builds, it manages to generate a standard memset of
zero values, but takes a long time doing so.

Call out to the `llvm.memset` function to zero out the memory instead.

src/librustc_trans/trans/intrinsic.rs

@@ -361,12 +361,11 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
         }
         (_, "init") => {
             let tp_ty = *substs.types.get(FnSpace, 0);
-            let lltp_ty = type_of::arg_type_of(ccx, tp_ty);
-            if return_type_is_void(ccx, tp_ty) {
-                C_nil(ccx)
-            } else {
-                C_null(lltp_ty)
+            if !return_type_is_void(ccx, tp_ty) {
+                // Just zero out the stack slot
+                zero_mem(bcx, llresult, tp_ty);
             }
+            C_nil(ccx)
         }
         // Effectively no-ops
         (_, "uninit") | (_, "forget") => {