Add an additional bit to ModRefInfo, MRI_Must, to be set for known must aliases.
Conservatively this bit will not be set (equivalent to MayAlias or NoAlias).
- MRI_Must is almost entirely set in the AAResults methods, the remaining changes are trying to preserve it.
- Only some small changes to make custom AA passes set MRI_Must (BasicAA).
- GlobalsModRef already declares a bit of the same value as MRI_Must (MayReadAnyGlobal). No changes to shift the value of MayReadAnyGlobal (see AlignedMap). FunctionInfo.getModRef() masks out everything except MRI_ModRef, so correctness is preserved, but the MRI_Must info is not set/used.
- There are cases where the MRI_Must is not set, e.g. 2 calls that only read will return MRI_NoModRef, though they may read from exactly the same location.