mir: Don't memset allocas of types that do not require drop.

Author: eddyb

src/librustc_trans/trans/mir/lvalue.rs

@@ -51,7 +51,9 @@ impl<'tcx> LvalueRef<'tcx> {
     {
         assert!(!ty.has_erasable_regions());
         let lltemp = bcx.with_block(|bcx| base::alloc_ty(bcx, ty, name));
-        drop::drop_fill(bcx, lltemp, ty);
+        if bcx.fcx().type_needs_drop(ty) {
+            drop::drop_fill(bcx, lltemp, ty);
+        }
         LvalueRef::new_sized(lltemp, LvalueTy::from_ty(ty))
     }
 }