[WIP] Build assume from call

Summary:
this is part of the implementation of http://lists.llvm.org/pipermail/llvm-dev/2019-December/137632.html

this patch gives the basis of building an assume to preserve all information from an instruction and add support for building an assume that preserve the information from a call.

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: mgrang, fhahn, mgorny, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72475