[Attributor] ValueSimplify Abstract Attribute

[Attributor] ValueSimplify Abstract Attribute

Summary:
This patch introduces initial `AAValueSimplify` which simplifies a value in a context.

example
- (for function returned) If all the return values are the same and constant, then we can replace callsite returned with the constant.
- If an internal function takes the same value(constant) as an argument in the callsite, then we can replace the argument with that constant.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66967

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371291 91177308-0d34-0410-b5e6-96231b3b80d8