[SCCP] Split up callsite handling, only propagate result on change (NFC)

Functions include their arguments in the use-list. Changed function
values mean that the result of the function changed. We only need
to update the call sites with the new function result and do not
have to propagate the call arguments.

To do so, this patch splits up the visitCallSite into handleCallResult
and handleCallArguments and updates markUsersAsChanged to only update
call results for functions.

Reviewers: efriedma, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D75846