Unique value write accesses

Ensure there is at most one write access per definition of an
llvm::Value. Keep track of already created value write access by using
a (dense) map.

Replace addValueWriteAccess by ensureValueStore which can be uses more
liberally without worrying to add redundant accesses. It will be used,
e.g. in a logical correspondant for value reads -- ensureValueReload --
to ensure that the expected definition has been written when loading it.

Differential Revision: http://reviews.llvm.org/D15483

llvm-svn: 258807