inalloca: Don't remove dead arguments in the presence of inalloca args

It disturbs the layout of the parameters in memory and registers,
leading to problems in the backend.

The plan for optimizing internal inalloca functions going forward is to
essentially SROA the argument memory and demote any captured arguments
(things that aren't trivially written by a load or store) to an indirect
pointer to a static alloca.

llvm-svn: 200717