ScopInfo: Never add read accesses for synthesizable values

Before adding a MK_Value READ MemoryAccess, check whether the read is
necessary or synthesizable. Synthesizable values are later generated by
the SCEVExpander and therefore do not need to be transferred
explicitly. This can happen because the check for synthesizability has
presumbly been forgotten in the case where a phi's incoming value has
been defined in a different statement.

Differential Revision: http://reviews.llvm.org/D15687

llvm-svn: 258998