[RISCV] Add combination crypto extensions in ISAInfo

The crypto extension have several shorthand extensions that don't consist of any extra instructions.
Take `zk` for example, while the extension would imply `zkn, zkr, zkt`. The 3 extensions should also
combine back into `zk` to maintain the canonical order in isa strings.

This patch addresses the above.

Reviewed By: VincentWu

Differential Revision: https://reviews.llvm.org/D119530