The "dead" markings allow existing target-independent optimizations, like MachineSink, to trigger more frequently. The CPSR defs would have eventually been marked dead by LiveVariables, so this only affects optimizations before regalloc.
The ARMBaseInstrInfo.cpp is fixing a bug which is only visible with this change: the transform adds a use to an otherwise dead def of CPSR. This is covered by existing regression tests.
Saves roughly 0.1% codesize on a Thumb1 codebase I've been looking at.
Loosely depends on D53452; one regression test related to TBB formation breaks without that change.