[Clang][CodeGen] Don't emit assumptions if current block is unreachable. (#106936)

Fixes #106898.

When emitting an infinite loop, clang codegen will delete the whole
block and leave builder's current block as nullptr:

https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/clang/lib/CodeGen/CGStmt.cpp#L597-L600

Then clang will create `zext (icmp slt %a, %b)` without parent block for
`a < b`. It will crash here:

https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/clang/lib/CodeGen/CGExprScalar.cpp#L416-L420

Even if we disabled this optimization, it still crashes in
`Builder.CreateAssumption`:

https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/llvm/lib/IR/IRBuilder.cpp#L551-L561

This patch disables assumptions emission if current block is null.

(cherry picked from commit c94bd96)

Author: dtcxzyw

clang/lib/CodeGen/CGStmt.cpp

@@ -745,7 +745,7 @@ void CodeGenFunction::EmitAttributedStmt(const AttributedStmt &S) {
     } break;
     case attr::CXXAssume: {
       const Expr *Assumption = cast<CXXAssumeAttr>(A)->getAssumption();
-      if (getLangOpts().CXXAssumptions &&
+      if (getLangOpts().CXXAssumptions && Builder.GetInsertBlock() &&
           !Assumption->HasSideEffects(getContext())) {
         llvm::Value *AssumptionVal = EvaluateExprAsBool(Assumption);
         Builder.CreateAssumption(AssumptionVal);

clang/test/SemaCXX/cxx23-assume.cpp

@@ -158,3 +158,12 @@ foo (int x, int y)
   return x + y;
 }
 }
+
+// Do not crash when assumptions are unreachable.
+namespace gh106898 {
+int foo () { 
+    while(1);
+    int a = 0, b = 1;
+    __attribute__((assume (a < b)));
+}
+}