[llvm] [AMDGPU] Speed up SIRegisterInfo::getReservedRegs (PR #79610)

[Jay Foad via llvm-commits]

jayfoad wrote:

Also for a future patch, I think it might make sense to change getReservedRegs so that it reserves everything by default (initializing the Reserved BitVector to all 1s) and then only un-reserves the available SGPRs/VGPRs/AGPRs.

https://github.com/llvm/llvm-project/pull/79610