[RISCV] Only check for scalar VT at depth 0 in hasAllNBitUsers.

VTs on already selected instructions can be arbitrary. Reviewing
the isel table I see i32 used for instructions that are part of
multiple instruction output patterns. Looks like tblgen to just
picks the lowest numbered MVT that is legal for the destination
register class of the instruction.

Seems better to just not check types for already selected nodes.