Rewrite init() interpretation to a real interpreter

Instead of mostly heuristics, actually execute the init() instruction in
an interpreter to calculate initializers for globals. This is far more
flexible and extensible, and gives the option of extending the
interpreter to other code and make it a partial evaluator.

ir.go

@@ -38,9 +38,10 @@ type Function struct {
 
 // Global variable, possibly constant.
 type Global struct {
-	g          *ssa.Global
-	llvmGlobal llvm.Value
-	flag       bool // used by dead code elimination
+	g           *ssa.Global
+	llvmGlobal  llvm.Value
+	flag        bool // used by dead code elimination
+	initializer Value
 }
 
 // Type with a name and possibly methods.