[llvm] [MachinePipeliner] Skip reserved registers when computing register pressure (PR #120694)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Hello @wangpc-pp 

After this patch the MF member seem to be unused:
```
../lib/CodeGen/MachinePipeliner.cpp:1219:26: error: private field 'MF' is not used [-Werror,-Wunused-private-field]
 1219 |   const MachineFunction &MF;
      |                          ^
1 error generated.
```

https://github.com/llvm/llvm-project/pull/120694