During the CodeGenPrepare we often lower intrinsics (such as objsize)

and allow some optimizations to turn conditional branches into unconditional.
This commit adds a simple control-flow optimization which merges two consecutive
basic blocks which are connected by a single edge. This allows the codegen to
operate on larger basic blocks.

rdar://11973998



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161852 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/CodeGenPrepare/basic.ll

@@ -5,7 +5,7 @@ target triple = "x86_64-apple-darwin10.0.0"
 
 ; CHECK: @test1
 ; objectsize should fold to a constant, which causes the branch to fold to an
-; uncond branch.
+; uncond branch. Next, we fold the control flow alltogether.
 ; rdar://8785296
 define i32 @test1(i8* %ptr) nounwind ssp noredzone align 2 {
 entry:
@@ -13,8 +13,8 @@ entry:
   %1 = icmp ugt i64 %0, 3
   br i1 %1, label %T, label %trap
 
-; CHECK: entry:
-; CHECK-NEXT: br label %T
+; CHECK: T:
+; CHECK-NOT: br label %
 
 trap:                                             ; preds = %0, %entry
   tail call void @llvm.trap() noreturn nounwind