Use sess.opts.optimize instead of sess.opts.cg.opt_level for LTO optlevel

Fixes rust-lang#22525

Author: rprichard

src/librustc_trans/back/lto.rs

@@ -167,7 +167,12 @@ pub fn run(sess: &session::Session, llmod: ModuleRef,
         llvm::LLVMRustAddAnalysisPasses(tm, pm, llmod);
         llvm::LLVMRustAddPass(pm, "verify\0".as_ptr() as *const _);
 
-        let opt = sess.opts.cg.opt_level.unwrap_or(0) as libc::c_uint;
+        let opt = match sess.opts.optimize {
+            config::No => 0,
+            config::Less => 1,
+            config::Default => 2,
+            config::Aggressive => 3,
+        };
 
         let builder = llvm::LLVMPassManagerBuilderCreate();
         llvm::LLVMPassManagerBuilderSetOptLevel(builder, opt);