[InstCombine] Improve check for catchswitch BBs (NFC)

[nikic]

LG

llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp

for (auto Incoming : zip(PN->blocks(), PN->incoming_values())) {
       // split the edge in instcombine, we have to bail out.
       return nullptr;
     }
 
     // If the incoming value is a PHI node before a catchswitch, we cannot
     // extract the value within that BB because we cannot insert any non-PHI
     // instructions in the BB.
     for (auto *Pred : PN->blocks())
-      if (isa<CatchSwitchInst>(Pred->getFirstNonPHI()))
+      if (Pred->getFirstInsertionPt() == Pred->end())
         return nullptr;
 
     for (User *U : PN->users()) {
       Instruction *UserI = cast<Instruction>(U);
 
       // If the user is a PHI, inspect its uses recursively.
       if (PHINode *UserPN = dyn_cast<PHINode>(UserI)) {
         if (PHIsInspected.insert(UserPN).second)