[CodeGen] llvm.allow.{runtime,ubsan}.check() in FastISel

Follow up to #86049.
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Author: vitalybuka

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1462,8 +1462,13 @@ bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
     llvm_unreachable("llvm.is.constant.* should have been lowered already");
 
   case Intrinsic::allow_runtime_check:
-  case Intrinsic::allow_ubsan_check:
-    llvm_unreachable("llvm.*.check should have been lowered already");
+  case Intrinsic::allow_ubsan_check: {
+    Register ResultReg = getRegForValue(ConstantInt::getTrue(II->getType()));
+    if (!ResultReg)
+      return false;
+    updateValueMap(II, ResultReg);
+    return true;
+  }
 
   case Intrinsic::launder_invariant_group:
   case Intrinsic::strip_invariant_group: