[Attributor] Use the proper context instruction in genericValueTraversal

There was a TODO in genericValueTraversal to provide the context
instruction and due to the lack of it users that wanted one just used
something available. Unfortunately, using a fixed instruction is wrong
in the presence of PHIs so we need to update the context instruction
properly.

Reviewed By: uenoku

Differential Revision: https://reviews.llvm.org/D76870