[RISCV] Restructure i1 insert/extract element costing [nfc-ish]

These get expanded through i8, so let's just model that directly instead of looking at the expanded code and trying to fit that into the flow. This results in slightly better costs for constant indices today (we can remove the add), but is mostly focused on making future changes easier.

Differential Revision: https://reviews.llvm.org/D158770