[RISCV][NFC] Unify compatibility checks under one function

Split off from D125021.

We were duplicating logic across different phases. Since we want to
ensure a consistency of logic across phases for correctness, this patch
combines our multiple compatibility checks into one function to better
convey this.

Several methods were made const too.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D126472