[RISCV] Change GPRMemAtomic to GPRMemZeroOffset for general usage

    Not only some AMO instructions but also other instructions need to
    process (${gpr}) or 0(${gpr}), where the 0 is be silently ignored.

    This patch does some changes for general usage.

Signed-off-by: Eric Tang <eric.tang@starfivetech.com>

Differential Revision: https://reviews.llvm.org/D120017