IR: Implement Value::mergeUseLists() iteratively

This avoids stack overflows when the the compiler does not perform tail call
elimination. Apparently this happens for MSVC with the /Ob2 switch which
may be used by external code including this header.

Reported by and based on a patch from Jean-Francois Riendeau.

Related to rdar://21900756

llvm-svn: 243590