compiler: add globaldce pass to start of optimization pipeline

This reduces code size in a few cases when tested against the drivers
smoketests (although there was one minor increase) without significantly
increasing compile time. In fact, in my testing compile time appears to
be going down a little bit (around 1%, within the noise).

compiler/optimizer.go

@@ -45,6 +45,7 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) []er
 		// Run some preparatory passes for the Go optimizer.
 		goPasses := llvm.NewPassManager()
 		defer goPasses.Dispose()
+		goPasses.AddGlobalDCEPass()
 		goPasses.AddGlobalOptimizerPass()
 		goPasses.AddConstantPropagationPass()
 		goPasses.AddAggressiveDCEPass()