[llvm] [NFC][AMDGPU] Compute always reserved registers once (PR #132006)

[Jay Foad via llvm-commits]

jayfoad wrote:

Why? Is this a compile time performance improvement? Is it measurable?

I know getReservedRegs is expensive, but the parts you have moved into getAlwaysReservedRegs look relatively cheap.

https://github.com/llvm/llvm-project/pull/132006