compiler: do not remove dead globals

These were not removed when they contain data, and will be removed by
LLVM anyway.

ir.go

@@ -50,7 +50,6 @@ type Function struct {
 type Global struct {
 	g           *ssa.Global
 	llvmGlobal  llvm.Value
-	flag        bool // used by dead code elimination
 	initializer Value
 }