[RISCV] Lower scalar_to_vector for supported FP types (#114340)

In https://reviews.llvm.org/D147608 we added custom lowering for
integers, but inadvertently also marked it as custom for scalable FP
vectors despite not handling it.

This adds handling for floats and marks it as custom lowered for
fixed-length FP vectors too.

Note that this doesn't handle bf16 or f16 vectors that would need
promotion, but these scalar_to_vector nodes seem to be emitted when
expanding them.