folding const expressions with branching logic (#24689)

motivating example:
```nim
iterator p(a: openArray[char]): int =
  if a.len != 0:
    if a[0] != '/':
      discard
for t in p(""): discard
```
The compiler wants to evaluate `a[0]` at compile time even though it is
protected by the if statement above it. Similarly expressions like
`a.len != 0 and a[0] == '/'` have problems. It seems like the logic in
semfold needs to be more aware of branches to positively identify when
it is okay to fail compilation in these scenarios. It's a bit tough
though because it may be the case that non-constant expressions in
branching logic can properly protect some constant expressions.

compiler/semfold.nim

@@ -471,19 +471,20 @@ proc foldArrayAccess(m: PSym, n: PNode; idgen: IdGenerator; g: ModuleGraph): PNo
       if result.kind == nkExprColonExpr: result = result[1]
     else:
       result = nil
-      localError(g.config, n.info, formatErrorIndexBound(idx, x.len-1) & $n)
+      #localError(g.config, n.info, formatErrorIndexBound(idx, x.len-1) & $n)
   of nkBracket:
     idx -= toInt64(firstOrd(g.config, x.typ))
     if idx >= 0 and idx < x.len: result = x[int(idx)]
     else:
       result = nil
-      localError(g.config, n.info, formatErrorIndexBound(idx, x.len-1) & $n)
+      #localError(g.config, n.info, formatErrorIndexBound(idx, x.len-1) & $n)
   of nkStrLit..nkTripleStrLit:
     result = newNodeIT(nkCharLit, x.info, n.typ)
     if idx >= 0 and idx < x.strVal.len:
       result.intVal = ord(x.strVal[int(idx)])
     else:
-      localError(g.config, n.info, formatErrorIndexBound(idx, x.strVal.len-1) & $n)
+      result = nil
+      #localError(g.config, n.info, formatErrorIndexBound(idx, x.strVal.len-1) & $n)
   else: result = nil
 
 proc foldFieldAccess(m: PSym, n: PNode; idgen: IdGenerator; g: ModuleGraph): PNode =

tests/controlflow/tcontrolflow.nim

@@ -114,3 +114,10 @@ block named: # works
   if true:
     break named
   doAssert false, "not reached"
+
+block:
+  iterator p(a: openArray[char]): int =
+    if a.len != 0:
+      if a[0] != '/':
+        discard
+  for t in p(""): discard