[Attributor] Use structured deduction for AADereferenceable

Summary:
This is analogous to D66128 but for AADereferenceable. We have the logic
concentrated in the floating value updateImpl and we use the combiner
helper classes for arguments and return values.

The regressions will go away with "on-demand" attribute creation.
Improvements are already visible in the existing tests.

Reviewers: uenoku, sstefan1

Subscribers: hiraditya, bollu, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66272

llvm-svn: 369329