[DeadArgumentElim] Set pointer to DISubprogram before calling RAUW. NFC

It is better to update pointer of the DISuprogram before we call RAUW for
still live arguments of the function, because with the change reviewed in
D42541 in RAUW we compare DISubprograms rather than functions itself.

Patch by Djordje Todorovic.

Differential Revision: https://reviews.llvm.org/D42794

llvm-svn: 324335