Currently vector splice intrinsics are costed through getShuffleCost
when the offset is fixed. When the offset is variable though we can't
use a shuffle mask so it currently returns invalid.
This implements the cost in RISCVTTIImpl::getIntrinsicInstrCost as the
cost of a slideup and a slidedown, which matches the codegen.
It also implements the type based cost whenever the offset argument
isn't available.
It may be possible to reduce the cost in future when one of the vector
operands is known to be poison, in which case we only generate a single
slideup or slidedown.
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