[RISCV] Add a special case to treat riscv-v-vector-bits-min=-1 as meaning use Zvl*b value.

riscv-v-vector-bits-min is primarily used to opt-in to the
autovectorizer. The vector width can be determined from Zvl*b.

This patch adds support treating -1 as meaning use Zvl*b so we can
still opt-in to autovectorization without needing to repeat a
vector width already given by Zvl*b or -mcpu.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D124960