[GuardWidening] Support widening of explicitly expressed guards

This patch adds support of guards expressed in explicit form via
`widenable_condition` in Guard Widening pass.

Differential Revision: https://reviews.llvm.org/D56075
Reviewed By: reames



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353932 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed