[RISCV] Verify the VL and Mask on the outer TRUNCATE_VECTOR_VL in combineTruncOfSraSext. (#93578)

We checked the VL and mask of any additional TRUNCATE_VECTOR_VL
nodes we peek through, but not the outermost.
    
This moves the check to the outer node and then verifies all the
additional nodes have the same VL and Mask.

Stacked on #93574