Store Arguments in a flat array instead of an iplist

This saves two pointers from Argument and eliminates some extra
allocations.

Arguments cannot be inserted or removed from a Function because that
would require changing its Type, which LLVM does not allow. Instead,
passes that change prototypes, like DeadArgElim, create a new Function
and copy over argument names and attributes. The primary benefit of
iplist is O(1) random insertion and removal. We just don't need that for
arguments, so don't use it.

Reviewed By: chandlerc

Subscribers: dlj, inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D31058

llvm-svn: 298105