[RISCV] Rewrite isLegalElementTypeForRVV in terms of ValueTypes [nfc]

This was briefly mentioned as a possibility in review discussion on D149369.

One slightly surprising bit to call out - these interfaces can get invoked with non-vector typed arguments. LoopVectorizer likes to call the costing interfaces with scalar types when unrolling, but not vectorizing. I found that surprising, not sure if others do.

Differential Revision: https://reviews.llvm.org/D149462