Current implementation of Value::replaceUsesExceptBlockAddr() uses UseList

iterator to walk the list which keeps changing inside the loop. When the
UseList contains several uses with the same user, we end processing the same
user more than once, which leads to an assert.

With this fix, unique users are saved and processed later to avoid
processing duplicates.

Differential Revision: https://reviews.llvm.org/D39864

llvm-svn: 318477