[llvm] [TRI] Remove reserved registers in getRegPressureSetLimit (PR #118787)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> > > Is there a compile time impact for this patch?
> > 
> > 
> > This should increase some compile-time, but I don't know if it is significant. The limits are cached in `RegisterClassInfo::getRegPressureSetLimit`, so it is the same for the users of this API; the limits will be calculated once in these direct users of `TargetRegisterInfo::getRegPressureSetLimit`, so it is not a performance/time-critical code path. cc @nikic @dtcxzyw Can you help me to meassure the compile time impact?
> 
> The limits aren’t cached for passes like MachineLICM right? And it will be recomputed for each function? My understanding of RegisterClassInfo is that it maintains the cache across functions as long as they have the same the subtarget or something like that?

Yes, you are right! I will force these passes to use `RegisterClassInfo ` as follow ups.

https://github.com/llvm/llvm-project/pull/118787