interp: fix inserting non-const values in a const aggregate

This bug was triggered by the following code:

    package main

    func foo() byte

    var array = [1]byte{foo()}

    func main() {
    }

interp/testdata/basic.ll

@@ -2,6 +2,8 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64--linux"
 
 @main.v1 = internal global i64 0
+@main.nonConst1 = global [4 x i64] zeroinitializer
+@main.nonConst2 = global i64 0
 
 declare void @runtime.printint64(i64) unnamed_addr
 
@@ -31,6 +33,20 @@ define internal void @main.init() unnamed_addr {
 entry:
   store i64 3, i64* @main.v1
   call void @"main.init#1"()
+
+  ; test the following pattern:
+  ;     func someValue() int // extern function
+  ;     var nonConst1 = [4]int{someValue(), 0, 0, 0}
+  %value1 = call i64 @someValue()
+  %gep1 = getelementptr [4 x i64], [4 x i64]* @main.nonConst1, i32 0, i32 0
+  store i64 %value1, i64* %gep1
+
+  ; Test that the global really is marked dirty:
+  ;     var nonConst2 = nonConst1[0]
+  %gep2 = getelementptr [4 x i64], [4 x i64]* @main.nonConst1, i32 0, i32 0
+  %value2 = load i64, i64* %gep2
+  store i64 %value2, i64* @main.nonConst2
+
   ret void
 }
 
@@ -40,3 +56,5 @@ entry:
   call void @runtime.printnl()
   ret void
 }
+
+declare i64 @someValue()