[RISCV] Add patterns for vnsrl.vx where shift amount is truncated

Similar to D155698 where the shift amount is extended, this patch extends the
ComplexPattern to handle the case where the shift amount has been truncated.
Truncations are custom lowered to truncate_vector_vl, and in cases like i64 ->
i16 they are truncated by one power of two at a time, so we need to unravel
nested layers of them.

The pattern can also be reused for Zvbb's vwsll.vx in an upcoming patch.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155928