[SDAG] Remove invalid check (NFC) (#146899)

It does not make sense to do !LC. LC is an RTLIB::Libcall enum, and zero
is a valid value.

Author: MacDue

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2519,8 +2519,11 @@ bool SelectionDAG::expandMultipleResultFPLibCall(
   EVT VT = Node->getValueType(0);
   unsigned NumResults = Node->getNumValues();
 
+  if (LC == RTLIB::UNKNOWN_LIBCALL)
+    return false;
+
   const char *LCName = TLI->getLibcallName(LC);
-  if (!LC || !LCName)
+  if (!LCName)
     return false;
 
   auto getVecDesc = [&]() -> VecDesc const * {