interp: fix "todo: store" panic

This is really just a simple workaround. When such an instruction is
encountered, it will just fall back to marking the entire function as
having side effects. Ideally it should trace all affected instructions
and check if they would have any side effects, but this at least fixes a
number of compile errors.

This commit gets the following packages to compile:

  * context
  * database/sql/driver
  * image/jpeg
  * image/png

interp/testdata/scan.ll

@@ -47,6 +47,12 @@ define i64 @readDirtyGlobal() {
 
 declare i64* @getDirtyPointer()
 
+define void @storeToPointer() {
+  %ptr = call i64* @getDirtyPointer()
+  store i64 3, i64* %ptr
+  ret void
+}
+
 @functionPointer = global i64()* null
 define i64 @callFunctionPointer() {
   %fp = load i64()*, i64()** @functionPointer