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/scan.go

@@ -193,10 +193,13 @@ func (e *Eval) hasLocalSideEffects(dirtyLocals map[llvm.Value]struct{}, inst llv
 				// Already handled in (*Eval).hasSideEffects.
 				continue
 			}
-			// But a store might also store to an alloca, in which case all uses
-			// of the alloca (possibly indirect through a GEP, bitcast, etc.)
-			// must be marked dirty.
-			panic("todo: store")
+			// This store might affect all kinds of values. While it is
+			// certainly possible to traverse through all of them, the easiest
+			// option right now is to just assume the worst and say that this
+			// function has side effects.
+			// TODO: traverse through all stores and mark all relevant allocas /
+			// globals dirty.
+			return true
 		default:
 			// All instructions that take 0 or more operands (1 or more if it
 			// was a use) and produce a result.

interp/scan_test.go

@@ -22,6 +22,7 @@ var scanTestTable = []struct {
 	{"readDirtyGlobal", sideEffectLimited, []string{"dirtyGlobalInt"}},
 	{"callFunctionPointer", sideEffectAll, []string{"functionPointer"}},
 	{"getDirtyPointer", sideEffectLimited, nil},
+	{"storeToPointer", sideEffectLimited, nil},
 }
 
 func TestScan(t *testing.T) {

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