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

[Tobias Stadler via llvm-commits]

https://github.com/tobias-stadler commented:

For the record, 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.

I'm fine with this change though, because this is a general problem with all other places where ObserverWrapper is used with only one Observer and I eventually want to rework this such that when only one Observer is in the ObserverWrapper, it is installed directly instead.

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