[AliasAnalysis] Add missing FMRB_* enums.

Previously, the enums didn't account for all the possible cases, which
could cause misleading results (particularly for a "switch" on
FunctionModRefBehavior).

Fixes regression in polly from recent patch to add writeonly to memset.

While I'm here, also fix a few dubious uses of the FMRB_* enum values.

Differential Revision: https://reviews.llvm.org/D73154