Ultimately I didn't introduce a custom SelectionDAG node, but instead added a DAG combine that inserts an AssertZext i5 on the shift amount for an i32 variable-length shift and also added an ANY_EXTEND DAG-combine which will instead produce a SIGN_EXTEND for an i32 variable-length shift, increasing the opportunity to safely select SLLW/SRLW/SRAW.
There are obviously different ways of addressing this (a number discussed in the llvm-dev thread), so I'd welcome further feedback and comments.
Note that there are now some cases in test/CodeGen/RISCV/rv64i-exhaustive-w-insts.ll where sraw/srlw/sllw is selected even though sra/srl/sll could be used without any extra instructions. Given both are semantically equivalent, there doesn't seem a good reason to prefer one vs the other. Given that would require more logic to still select sra/srl/sll in those cases, I've left it preferring the *w variants.