Don't run the DCE pass when optimization is disabled.

The main purpose of the DCE pass is to clean up dead code left behind by
the optimizer, so it's not valuable to run it when the optimizer isn't
being run.

lib/cretonne/src/context.rs

@@ -105,7 +105,9 @@ impl Context {
         }
         self.compute_domtree();
         self.eliminate_unreachable_code(isa)?;
-        self.dce(isa)?;
+        if isa.flags().opt_level() != OptLevel::Fastest {
+            self.dce(isa)?;
+        }
         self.regalloc(isa)?;
         self.prologue_epilogue(isa)?;
         self.relax_branches(isa)