ARC operations don't have an effect on immortal objects, like the empty array singleton or statically allocated arrays.
Therefore we can freely remove and retain/release instructions on such objects, even if there is no paired balanced ARC operation.
This optimization can only be done with a minimum deployment target of Swift 5.1, because in that version we added immortal ref count bits.
The optimization is implemented in libswift. Additionally, the remaining logic of simplifying strong_retain and strong_release is also ported to libswift.
rdar://81482156
* unify FunctionPassContext and InstructionPassContext
* add a modification API: PassContext.setOperand
* automatic invalidation notifications when the SIL is modified
