Optimize trivial branches in CodeGenPrepare, which often get created from the

lowering of objectsize intrinsics. Unfortunately, a number of tests were relying
on llc not optimizing trivial branches, so I had to add an option to allow them
to continue to test what they originally tested.

This fixes <rdar://problem/8785296> and <rdar://problem/9112893>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127459 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/CodeGenPrepare/basic.ll

@@ -14,13 +14,14 @@ entry:
   br i1 %1, label %T, label %trap
 
 ; CHECK: entry:
-; HECK-NEXT: ret i32 4
+; CHECK-NEXT: br label %T
 
 trap:                                             ; preds = %0, %entry
   tail call void @llvm.trap() noreturn nounwind
   unreachable
 
 T:
+; CHECK: ret i32 4
   ret i32 4
 }