[RISCV] Move the custom isel for (add X, imm) into SelectAddrRegImm.

This custom isel was used to split the lo12 bits of the imm so that
they could be folded into load/store addresses via a post-isel
peephole.

This patch instead splits the immediate during isel and folds the
lo12 removing the need for the post-isel peephole to do anything.

After this we'll be able to remove the post-isel peephole.

Reviewed By: asb, luismarques

Differential Revision: https://reviews.llvm.org/D129450