[SR-13668] SILOptimizer: devirtualize `differentiability_witness_function` instructions

[dan-zheng]

Previous ID	SR-13668
Radar	None
Original Reporter	@dan-zheng
Type	Sub-task
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s
Labels	Sub-task
Assignee	@marcrasi
Priority	Medium

md5: 5f602f3bd32262763c28fdde561d04a4

Parent-Task:

• SR-13659 Retroactive derivative registration

Issue Description:

Devirtualizing differentiability_witness_function instructions to function_ref should positively impact performance.

Reference: lib/SILOptimizer/Utils/Devirtualize.cpp handles class_method and witness_method instructions.

[dan-zheng]

Done in #28480

A new "DifferentiabilityWitnessDevirtualizer" function transform was created. This was simpler than hooking into swift::tryDevirtualizeApply: #28480 (comment)