Add dead code elimination (DCE) pass

This should make it much easier (in the future) to use the standard
library when unused functions contain unimplemented features. But more
importantly, it makes later passes better and makes compiling faster by
not having to scan dead code.

ir.go

@@ -31,6 +31,7 @@ type Function struct {
 	fn       *ssa.Function
 	llvmFn   llvm.Value
 	blocking bool
+	flag     bool        // used by dead code elimination
 	parents  []*Function // calculated by AnalyseCallgraph
 	children []*Function
 }
@@ -39,6 +40,7 @@ type Function struct {
 type Global struct {
 	g          *ssa.Global
 	llvmGlobal llvm.Value
+	flag       bool // used by dead code elimination
 }
 
 // Type with a name and possibly methods.