[DeadArgumentElimination] Preserve llvm.dbg.values's first argument

When removing return value Dead Argument Elimination pass clobbers first
llvm.dbg.value’s argument for live arguments of that function by replacing
it with nullptr. In the next pass it will be deleted, so debug location
about those arguments are lost. This change fixes it.

Patch by Djordje Todorovic.

Differential Revision: https://reviews.llvm.org/D42541

llvm-svn: 323784