[ScopInfo] Keep scalar acceess dictionaries up-to-data. NFC.

When removing a MemoryAccess, also remove it from maps pointing to it.
This was already done for InstructionToAccess, but not yet for
ValueReads, ValueWrites and PHIWrites as those were only used during
the ScopBuilder phase. Keeping them updated allows us to use them
later as well.

llvm-svn: 302836