OptimizeAutoreleaseRVCall skips optimizing llvm.objc.autoreleaseReturnValue if it
sees a user which is llvm.objc.retainAutoreleasedReturnValue, and if they have
equivalent arguments (either identical or equivalent PHIs). It then assumes that
ObjCARCOpt::OptimizeRetainRVCall will optimize the pair instead.
Trouble is, ObjCARCOpt::OptimizeRetainRVCall doesn't know about equivalent PHIs
so optimizes in a different way and we are left with an unoptimized llvm.objc.autoreleaseReturnValue.
This teaches ObjCARCOpt::OptimizeRetainRVCall to also understand PHI equivalence.
rdar://problem/47005143
Reviewed By: ahatanak
Differential Revision: https://reviews.llvm.org/D56235
llvm-svn: 350284
