[RISCV] Bitcast fixed length bf16/f16 build_vector to i16 with...

[RISCV] Bitcast fixed length bf16/f16 build_vector to i16 with Zvfbfmin/Zvfhmin+Zfbfmin/Zfhmin. (#106637)

Previously, if Zfbfmin/Zfhmin were enabled, we only handled
build_vectors that could be turned into splat_vectors. We promoted them
to f32 splats by extending in the scalar domain and narrowing in the
vector domain.

This patch fixes a crash where we failed to account for whether the f32
vector type fit in LMUL<=8.

Because the new lowering occurs after type legalization, we have to be
careful to use XLenVT for the scalar integer type and use custom cast
nodes.