[RISCV] Split TuneShiftedZExtFusion (#76032)

We split `TuneShiftedZExtFusion` into three fusions to make them
reusable and match the GCC implementation[1].

The zexth/zextw fusions can be reused by XiangShan[2] and other
commercial processors, but shifted zero extension is not so common.

`macro-fusions-veyron-v1.mir` is renamed so it's not relevant to
specific processor.

References:
[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-November/637303.html
[2] https://xiangshan-doc.readthedocs.io/zh_CN/latest/frontend/decode