[llvm] [RISCV][NFCI] Introduce Base for Pseudo Expansion Passes (PR #218170)

[Craig Topper via llvm-commits]

topperc wrote:

> This seems to have found a bug in our codegen for P, I'll fix that in a separate PR (the Post-RA inst expansion was not checking instruction sizes)

That pass doesn't really need to check size. Only expansions after BranchRelaxation need to check size.

https://github.com/llvm/llvm-project/pull/218170