[llvm] InstructionSelect: Use GISelChangeObserver instead of MachineFunction::Delegate (PR #105725)

[Daniel Sanders via llvm-commits]

dsandersllvm wrote:

Those changes are all done. I remember there was a conversation on discourse a few months back about clicking the resolve buttons but I'm not actually sure what the outcome was. I saw you were against the ban so I've resolved them

> I'm fine with this change though.

Thanks

> (For future reference: This causes a 0.05% geomean compile-time (instruction count) regression on CTMark AArch64 O0, which isn't negligible. This was the original reasoning for not using the ObserverWrapper. This is a more general issue with the ObserverWrapper. I eventually want to rework this such that when the ObserverWrapper contains only one Observer, it is installed without indirection to avoid this overhead.)

Our downstream usage of the delegate is quite localized to a couple intrinsics so hopefully we can remove it. If we manage that I'll come back to revert this commit

https://github.com/llvm/llvm-project/pull/105725