[Attributor] Implement "norecurse" function attribute deduction

Summary:
This patch introduces `norecurse` function attribute deduction.

`norecurse` will be deduced if the following conditions hold:
* The size of SCC in which the function belongs equals to 1.
* The function doesn't have self-recursion.
* We have `norecurse` for all call site.

To avoid a large change, SCC is calculated using scc_iterator in InfoCache initialization for now.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67751

llvm-svn: 372475