[VPlan] Use ExitBocks to check in VPlan::isExitBlock (NFC).

Exit blocks of the VPlan are now hold in ExitBlocks. Use it to check if
a block is an exit block. Otherwise we currently mis-classify the scalar
loop header also as exit block, as it is not explicitly connected to the
exit blocks.

NFC at the moment, as the helper currently is never queried with the
scalar header, but that will change in the future.

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -921,7 +921,7 @@ VPIRBasicBlock *VPlan::getExitBlock(BasicBlock *IRBB) const {
 }
 
 bool VPlan::isExitBlock(VPBlockBase *VPBB) {
-  return isa<VPIRBasicBlock>(VPBB) && VPBB->getNumSuccessors() == 0;
+  return is_contained(ExitBlocks, VPBB);
 }
 
 /// Generate the code inside the preheader and body of the vectorized loop.