apply of differentiable_function is not optimized

[asl]

Consider the testcase from #65073

Now we compile it down to:

..
  // function_ref closure #1 in f(array:)
  %7 = function_ref @$s6arrupd1f5arraySdSaySdG_tFySdzcfU_ : $@convention(thin) (@inout Double) -> () // user: %10
  // function_ref forward-mode derivative of closure #1 in f(array:)
  %8 = function_ref @$s6arrupd1f5arraySdSaySdG_tFySdzcfU_TJfSpSr : $@convention(thin) (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () // user: %11
  // function_ref reverse-mode derivative of closure #1 in f(array:)
  %9 = function_ref @$s6arrupd1f5arraySdSaySdG_tFySdzcfU_TJrSpSr : $@convention(thin) (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () // user: %12
  %10 = thin_to_thick_function %7 : $@convention(thin) (@inout Double) -> () to $@noescape @callee_guaranteed (@inout Double) -> () // user: %13
  %11 = thin_to_thick_function %8 : $@convention(thin) (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () // user: %14
  %12 = thin_to_thick_function %9 : $@convention(thin) (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () // user: %15
  %13 = convert_function %10 : $@noescape @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Double> // user: %16
  %14 = convert_function %11 : $@noescape @callee_guaranteed (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> (@owned @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>) for <Double> // user: %16
  %15 = convert_function %12 : $@noescape @callee_guaranteed (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> (@owned @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>) for <Double> // user: %16
  %16 = differentiable_function [parameters 0] [results 0] %13 : $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Double> with_derivative {%14 : $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> (@owned @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>) for <Double>, %15 : $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> (@owned @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>) for <Double>} // users: %95, %18
...
  %95 = apply %16(%94) : $@differentiable(reverse) @noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Double>

Note that we can fold differentiable_function here enabling further simplifications and inlining of $s6arrupd1f5arraySdSaySdG_tFySdzcfU_

[asl]

Tagging @BradLarson @rxwei @dan-zheng