[CodeGen] Add a pass to fold null checks into nearby memory operations.

[CodeGen] Add a pass to fold null checks into nearby memory operations.

Summary:
This change adds an "ImplicitNullChecks" target dependent pass.  This
pass folds null checks into memory operation using the FAULTING_LOAD
pseudo-op introduced in previous patches.

Depends on D10197
Depends on D10199
Depends on D10200

Reviewers: reames, rnk, pgavlin, JosephTremoulet, atrick

Reviewed By: atrick

Subscribers: ab, JosephTremoulet, llvm-commits

Differential Revision: http://reviews.llvm.org/D10201

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239743 91177308-0d34-0410-b5e6-96231b3b80d8