[RISCV] Add a test showing an incorrect vsetvli insertion

This patch adds a reduced test case which identifies an illegal vsetvli
inserted by the compiler. The compiler emits a vsetvli which is intended
to preserve VL with the SEW/LMUL ratio e32/m1 when in fact the VL could
have been set by e64/m2 in a predecessor block.

Differential Revision: https://reviews.llvm.org/D106286