[RISCV] Fold (add X, [-4096, -2049]) or (add X, [2048,4096]) into load/store address during isel.

Previously we iseled this to a pair of ADDIs and relied on a post
isel peephole to fold one of the ADDIs into the load/store. Now
we split the immediate in two parts the same way isel does and fold
one of the pieces. If the add has a non-memory use it will emit
two isels and larger one will CSE with the ADDI we created for the
the memory use.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D128741