[RISCV] Make computeIncomingVLVTYPE more conservative when merging predecessor state.

[craig.topper]

Summary

If we have already calculated the incoming state before, use that
as our starting point to ensure we are conservative.

[reames]

LGTM

For context, Craig and I debugged this earlier today. We had an infinite loop issue which came down to the fact we allowed two waves of updates to propagate through a loop, and the merge points allowed us to toggle back and forth between states instead of falling to the bottom value of the lattice. No known small reproducer at this time unfortunately.

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

 }
 
 void RISCVInsertVSETVLI::computeIncomingVLVTYPE(const MachineBasicBlock &MBB) {
 
   BlockData &BBInfo = BlockInfo[MBB.getNumber()];
 
   BBInfo.InQueue = false;
 
-  VSETVLIInfo InInfo;
+  // Start with the previous entry so that we keep the most conservative state
+  // we have ever found.
+  VSETVLIInfo InInfo = BBInfo.Pred;
   if (MBB.pred_empty()) {
     // There are no predecessors, so use the default starting status.
     InInfo.setUnknown();
   } else {
     for (MachineBasicBlock *P : MBB.predecessors())
       InInfo = InInfo.intersect(BlockInfo[P->getNumber()].Exit);
   }