[RISCV] Teach RISCVMergeBaseOffset to merge %lo/%pcrel_lo into load/store after folding arithmetic.

It's possible we have:
lui  a0, %hi(sym)
addi a0, %lo(sym)
addi a0, <offset1>
lw a0, <offset2>(a0)

We want to arrive at
lui a0, %hi(sym+offset1+offset2)
lw a0, %lo(sym+offset1+offset2)

We currently fail to do this because we only consider loads/stores
if we didn't find any arithmetic.

This patch splits arithmetic folding and load/store folding into
two separate phases. The load/store folding can no longer assume
the offset in hi/lo is 0 so we must combine the offsets. I've applied
the same simm32 limit that we applied in the arithmetic folding.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D130931