[RISCV] Teach RISCVMergeBaseOffset to handle read-modify-write of a global.

The pass was previously limited to LUI+ADDI being used by a single
instruction.

This patch allows the pass to optimize multiple memory operations
that use the same offset. Each of them will receive a separate %lo
relocation. My main motivation is to handle a read-modify-write
where we have a load and store to the same address, but I didn't
restrict it to that case.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D128599