[clang] [clang][bytecode] Fix 'if consteval' in non-constant contexts (PR #104707)

[Timm Baeder via cfe-commits]

================
@@ -4368,8 +4363,19 @@ template <class Emitter> bool Compiler<Emitter>::visitIfStmt(const IfStmt *IS) {
     if (!visitDeclStmt(CondDecl))
       return false;
 
-  if (!this->visitBool(IS->getCond()))
-    return false;
+  // Compile condition.
+  if (IS->isNonNegatedConsteval()) {
+    if (!this->emitIsConstantContext(IS))
+      return false;
+  } else if (IS->isNegatedConsteval()) {
+    if (!this->emitIsConstantContext(IS))
+      return false;
+    if (!this->emitInv(IS))
+      return false;
+  } else {
+    if (!this->visitBool(IS->getCond()))
+      return false;
+  }
----------------
tbaederr wrote:

I understand  your concern but I think this is conceptually no different than
```c++
constexpr int f(bool b) {
if (!b) {
  goto a;
  a:;
} else {
}
  return  1;
}
```
where the current interpreter would also only evaluate one of the branches while the bytecode interpreter needs to generate code for both.

https://github.com/llvm/llvm-project/pull/104707