[llvm] [RISCV] Set AllocationPriority in line with LMUL (PR #131176)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

This seems to increase the overall LMUL spilled on 538.imagick_r:

```
Program                                       riscv-instr-info.TotalLMULSpilled               riscv-instr-info.TotalLMULReloaded              
                                              lhs                               rhs     diff  lhs                                rhs     diff 
FP2017rate/538.imagick_r/538.imagick_r        4239.00                           5082.00 19.9% 6697.00                            7321.00  9.3%
FP2017speed/638.imagick_s/638.imagick_s       4239.00                           5082.00 19.9% 6697.00                            7321.00  9.3%
INT2017spe...31.deepsjeng_s/631.deepsjeng_s    132.00                            134.00  1.5%  274.00                             248.00 -9.5%
INT2017rat...31.deepsjeng_r/531.deepsjeng_r    132.00                            134.00  1.5%  274.00                             248.00 -9.5%
INT2017rate/520.omnetpp_r/520.omnetpp_r          4.00                              4.00  0.0%    5.00                               5.00  0.0%
INT2017speed/625.x264_s/625.x264_s              83.00                             83.00  0.0%   93.00                              93.00  0.0%
INT2017spe...23.xalancbmk_s/623.xalancbmk_s      6.00                              6.00  0.0%    6.00                               6.00  0.0%
INT2017spe...ed/620.omnetpp_s/620.omnetpp_s      4.00                              4.00  0.0%    5.00                               5.00  0.0%
INT2017speed/602.gcc_s/602.gcc_s                85.00                             85.00  0.0%   91.00                              91.00  0.0%
INT2017spe...00.perlbench_s/600.perlbench_s      4.00                              4.00  0.0%    8.00                               8.00  0.0%
INT2017rate/525.x264_r/525.x264_r               83.00                             83.00  0.0%   93.00                              93.00  0.0%
INT2017rat...23.xalancbmk_r/523.xalancbmk_r      6.00                              6.00  0.0%    6.00                               6.00  0.0%
FP2017rate/508.namd_r/508.namd_r                 1.00                              1.00  0.0%    6.00                               6.00  0.0%
FP2017rate/510.parest_r/510.parest_r          1084.00                           1084.00  0.0% 1368.00                            1368.00  0.0%
INT2017rat...00.perlbench_r/500.perlbench_r      4.00                              4.00  0.0%    8.00                               8.00  0.0%
FP2017speed/644.nab_s/644.nab_s                 25.00                             25.00  0.0%   25.00                              25.00  0.0%
FP2017speed/619.lbm_s/619.lbm_s                 38.00                             38.00  0.0%   38.00                              38.00  0.0%
FP2017rate/544.nab_r/544.nab_r                  25.00                             25.00  0.0%   25.00                              25.00  0.0%
FP2017rate/519.lbm_r/519.lbm_r                  38.00                             38.00  0.0%   38.00                              38.00  0.0%
FP2017rate/511.povray_r/511.povray_r           121.00                            121.00  0.0%  138.00                             138.00  0.0%
INT2017rate/502.gcc_r/502.gcc_r                 85.00                             85.00  0.0%   91.00                              91.00  0.0%
FP2017rate/526.blender_r/526.blender_r        1159.00                           1155.00 -0.3% 1292.00                            1298.00  0.5%
```

With that said though there's no actual impact on the runtime performance of that benchmark so I presume these spills are on the cold path. I looked at the codegen changes and the code there is pretty bad to begin with anyway.

I'm just flagging this FYI, I'm happy for this to land in any case.

https://github.com/llvm/llvm-project/pull/131176