Merge pull request #726 from rtfeldman/llvm-dce

DCE zig builtins

compiler/gen/src/llvm/build.rs

@@ -386,6 +386,9 @@ pub fn construct_optimization_passes<'a>(
     fpm.add_instruction_combining_pass();
     fpm.add_tail_call_elimination_pass();
 
+    // remove unused global values (e.g. those defined by zig, but unused in user code)
+    mpm.add_global_dce_pass();
+
     let pmb = PassManagerBuilder::create();
     match opt_level {
         OptLevel::Normal => {