[RISCV] Add RISCVISD::SLLW to computeKnownBitsForTargetNode.

Found while investigating whether we still need to stop DAG combiner
from turning (i64 (sext (i32 X))) into zext when i32 is known non
negative.

No test case because I still need to find fixes for some other issues
before I can remove the code from DAGCombiner.