[RISCV] Insert VSETVLI at the end of a basic block if we didn't produce BlockInfo.Exit.

This is an alternative to D118667 that instead of fixing the store
to match phase 1, it tries to detect the mismatch with the expected
value at the end of the block. This inserts a vsetvli after the vse
to satisfy the requirement of the other basic block.

We still have serious design issues in the pass, that is going to
require some rethinking.

Differential Revision: https://reviews.llvm.org/D119518