[ModRefInfo] Return NoModRef for Must and NoModRef.

Summary:
In ModRefInfo "Must" was introduced to track presence of MustAlias, but we still want to return NoModRef when there is neither Mod or Ref, even when MustAlias is found. Patch has small fixes to ensure this happens.
Minor cleanup to remove nesting for 2 if statements when calling getModRefInfo for 2 ImmutableCallSites.

Reviewers: sanjoy

Subscribers: jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D42209

llvm-svn: 322932