Modify ModRefInfo values using static inline method abstractions [NFC].

Summary:
The aim is to make ModRefInfo checks and changes more intuitive
and less error prone using inline methods that abstract the bit operations.

Ideally ModRefInfo would become an enum class, but that change will require
a wider set of changes into FunctionModRefBehavior.

Reviewers: sanjoy, george.burgess.iv, dberlin, hfinkel

Subscribers: nlopes, llvm-commits

Differential Revision: https://reviews.llvm.org/D40749

llvm-svn: 319821