The patch enhances sil-combiner handling of `differentiable_function` by
adding support for `convert_function` which is further used in
`differentiable_function_extract`:
```
%0 = differentiable_function ... %x
%1 = begin_borrow %0
%2 = convert_function %1 to ...
%3 = differentiable_function_extract [xxx] %2
// use of %3
```
-->
```
%0 = differentiable_function ... %x
// use of %x
```
The patch implements proper sil-combiner handling of
`differentiable_function` for cases when extractee has non-trivial
ownership. In such casese, it is consumed by the differentiable_function
instruction. We must copy the extractee before the consumption point so
the copy remains live afterward.
Fixes#88816
Support folding `differentiable_function_extract` of
`differentiable_function` in presence of borrowed scopes. Such folding
is crucial for VJP inlining, which is required for AutoDiff closure
specialization (ADCS) pass working properly.
Such handling was not required previously, but now ADCS runs in presence
of OSSA, making handling of borrowed scopes essential.
The folding logic is based on similar logic for
`struct`/`struct_extract` simplification.
Note that the `AutoDiff/SILOptimizer/licm_context.swift` test needs to
be modified since it relies on specific inlining behavior. Particularly,
we need to force inlining of the implicitly generated VJP of `B.a()`
into the VJP of `q()`. Without `@inline(__always)`, this particular
inlining decision stops happening on MacOS because the SIL combiner
changes from this patch affect the inlining decisions.
The changes from this patch make some new inlining decisions possible to
be taken befor attempting to inline the VJP of `B.a()` into the VJP of
`q()`. As a result, the VJP of `B.a()` becomes bigger because of other
VJPs being inlined into that, and the inlining cost of `B.a()` VJP
becomes too high when trying to perform inlining inside the VJP of
`q()`.
Depends on #87859 to allow force-inlining in
`AutoDiff/SILOptimizer/licm_context.swift`.
Daniil Kovalev