[CodeGen] Add a pass to fold null checks into nearby memory operations.
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.
Reviewers: reames, rnk, pgavlin, JosephTremoulet, atrick
Reviewed By: atrick
Subscribers: ab, JosephTremoulet, llvm-commits
Differential Revision: http://reviews.llvm.org/D10201