Allow call-slop optzn for destinations with a suitable dereferenceable attribute

Summary:
Currently, call slot optimization requires that if the destination is an
argument, the argument has the sret attribute. This is to ensure that
the memory access won't trap. In addition to sret, we can also allow the
optimization to happen for arguments that have the new dereferenceable
attribute, which gives the same guarantee.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5832