[RISCV] Unify depedency check and extension implication parsing logics

Originially there are two places that does parsing - `parseArchString` and
`parseFeatures`, each with its code on dependency check and implication.
This patch extracts common parts of the two  as functions of `RISCVISAInfo`
and let them 2 use it.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D112359