[ArgumentPromotion] Change use of removed argument in llvm.dbg.value to undef

Summary:
This solves PR33641.

When removing a dead argument we must also handle possibly existing calls
to llvm.dbg.value that use the removed argument. Now we change the use
of the otherwise dead argument to an undef for some other pass to cleanup
later.

If the calls are left untouched, they will later on cause errors:
 "function-local metadata used in wrong function"
since the ArgumentPromotion rewrites the code by creating a new function
with the wanted signature, but the metadata is not recreated so the new
function may then erroneously use metadata from the old function.

Reviewers: mstorsjo, rnk, arsenm

Reviewed By: rnk

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D34874

llvm-svn: 307521