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

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

This fixes an infinite loop found in our downstream where we
we allowed two waves of updates to propagate through a loop and
the merge points allowed us to toggle back and forth between states.
No small reproducer right now.

Differential Revision: https://reviews.llvm.org/D134229