[cleanup] Add a getOperandNo method to the Use class and implement it

out-of-line so that it can refer to the methods on User. As
a consequence, this removes the need to define one template method if
value_use_iterator in the extremely strange User.h header (!!!).

This makse Use.h slightly less peculiar. The only remaining real
peculiarity is the definition of Use::set in Value.h

llvm-svn: 202805