[RISCV][1/3] Switch undef -> poison in VP RVV tests

Inspired by a recent Discourse post on undef vs. poison usage, this
series of patches should reduce the number of undefs in LLVM tests by
around 10%.

Only undef vector operands to insertelement/shufflevector have been
handled, which are by far the most common we've got.

The switchover is split into 3 fairly arbitrary clusters to make it
slightly more manageable: vector predication, fixed-length vectors,
scalable vectors.