[RISCV] Remove RISCVMergeBaseOffsetOpt from the -O0 pass pipeline.

Internally the pass skips any function with the optnone attribute. But that still requires checking each function. If the opt level is set to None we might as well just skip putting in the pipeline at all. This what is already done for many of the passes added by TargetPassConfig.

Differential Revision: https://reviews.llvm.org/D92511