[RISCV] Start vslide1down sequence with a dependency breaking splat (#72691)

If we are using entirely vslide1downs to initialize an otherwise undef
vector, we end up with an implicit_def as the source of the first
vslide1down. This register has to be allocated, and creates false
dependencies with surrounding code.

Instead, start our sequence with a vmv.v.x in the hopes of creating a
dependency breaking idiom. Unfortunately, it's not clear this will
actually work as due to the VL=0 special case for T.A. the hardware has
to work pretty hard to recognize that the vmv.v.x actually has no source
dependence. I don't think we can reasonable expect all hardware to have
optimized this case, but I also don't see any downside in prefering it.