[AlwaysInliner] Make legacy pass like the new pass

The legacy pass is only used in AMDGPU codegen, which doesn't care about running it in call graph order (it actually has to work around that fact).

Make the legacy pass a module pass and share code with the new pass.

This allows us to remove the legacy inliner infrastructure.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D146446