Avoid creating llenv blocks when there's nothing to load

Currently, all closures have an llenv block to load values from the
captured environment, but for closure that don't actually capture
anything, that block is useless and can be skipped.

Author: dotdash

src/librustc/middle/trans/closure.rs

@@ -326,6 +326,11 @@ pub fn load_environment(fcx: fn_ctxt,
                         sigil: ast::Sigil) {
     let _icx = push_ctxt("closure::load_environment");
 
+    // Don't bother to create the block if there's nothing to load
+    if cap_vars.len() == 0 && !load_ret_handle {
+        return;
+    }
+
     let llloadenv = match fcx.llloadenv {
         Some(ll) => ll,
         None => {