[SelectionDAG][RISCV] Use FP type for legality query for LRINT/LLRINT in...

[SelectionDAG][RISCV] Use FP type for legality query for LRINT/LLRINT in LegalizeVectorOps. (#82728)

This matches how LRINT/LLRINT is queried for scalar types in
LegalizeDAG.

It's confusing if they do different things since a "Legal" vector
LRINT/LLRINT would get through to LegalizeDAG which would then consider
it illegal. This doesn't happen currently because RISC-V uses Custom.