[RISCV] Move vnclipu patterns into DAGCombiner. (#93596)

I plan to add support for multiple layers of vnclipu. For example,
i32->i8 using 2 vnclipu instructions. First clipping to 65535, then
clipping to 255. Similar for signed vnclip.
    
This scales poorly if we need to add patterns with 2 or 3 truncates.
Instead, move the code to DAGCombiner with new ISD opcodes to represent
VCLIP(U).
    
This patch just moves the existing patterns into DAG combine. Support
for multiple truncates will as a follow up. A similar patch series will
be made for the signed vnclip.