[llvm] [RISCV] Add a pass to remove ADDI by reassociating to fold into load/store address. (PR #127151)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

LGTM. Here are the results I get for dynamic instruction count impact as measured via qemu-user plugin (SPEC 2017, train dataset, -march=rv64gc and no LTO). Matches your comment on the leela_r impact, but there are gains for nab_r and xz_r too:

```
500.perlbench_r         174012006613    173921196845     -0.05%
502.gcc_r               218650852693    218971034713      0.15%
505.mcf_r               131573660594    131203488777     -0.28%
508.namd_r              217509594816    217412978405     -0.04%
510.parest_r            289306119855    289304644615     -0.00%
511.povray_r             31637559090     31636827075     -0.00%
519.lbm_r                95897914890     95897914899      0.00%
520.omnetpp_r           134722898826    134565059708     -0.12%
523.xalancbmk_r         281039434219    281025840539     -0.00%
525.x264_r              380167866567    380055777956     -0.03%
526.blender_r           659969371927    659746540280     -0.03%
531.deepsjeng_r         352331420401    349545945804     -0.79%
538.imagick_r           238558760759    238558763924      0.00%
541.leela_r             421624218698    406574474471     -3.57%
544.nab_r               414398714724    401491106871     -3.11%
557.xz_r                131583790604    130014971282     -1.19%
```

https://github.com/llvm/llvm-project/pull/127151