In AnalyzeCallGraph, if we treat InaccessibleMemOnly the same as ReadNone,
the function will end up having "FMRB_DoesNotAccessMemory". This will lead
to situations such as LICM moving out / eliminating calls such as malloc
(in the future, since malloc does not have InaccessibleMemOnly set yet)
Instead, treat InaccessibleMemOnly such that the analysis still maintains
info for the function, but keeps it conservative as ModRef.
Depends on D15605