[DeadArgElim] Set unused arguments for internal functions

[DeadArgElim] Set unused arguments for internal functions

Prior to this patch we would only set to undef the unused arguments of the
external functions. The rationale was that unused arguments of internal
functions wouldn't need to be turned into undef arguments because they
should have been simply eliminated by the time we reach that code.

This is actually not true because there are plenty of cases where we can't
remove unused arguments. For instance, if the internal function is used in
an indirect call, it may not be possible to change the function signature.
Yet, for statically known call-sites we would still like to mark the unused
arguments as undef.

This patch enables the "set undef arguments" optimization on internal
functions when we encounter cases where internal functions cannot be
optimized. I.e., whenever an internal function is marked "live".

Differential Revision: https://reviews.llvm.org/D124699