[RewriteStatepointsForGC] Strip deref info after rewriting.

[RewriteStatepointsForGC] Strip deref info after rewriting.

Summary:
Once a gc.statepoint has been rewritten to relocate live references, the
SSA values represent physical pointers instead of logical references.
Logical dereferencability does not imply physical dereferencability and
after RewriteStatepointsForGC has run any attributes that imply
dereferencability of the logical references need to be stripped.

This current approach is conservative, and can be made more precise
later if needed.  For starters, we need to strip dereferencable
attributes only from pointers that live in the GC address space.

Reviewers: reames, pgavlin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10105

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238883 91177308-0d34-0410-b5e6-96231b3b80d8