[RISCV] Match RISCVISD::ADD_LO in SelectAddrRegImm.

This allows us to fold global and constant pool addresses into
load/store during isel instead of in the post-isel peephole. I
did not copy the alignment check for ConsantPoolSDNode because it
wasn't tested.

This is a step towards being able to remove the post-isel
peephole.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D128738