fix rdar://8785296 - -fcatch-undefined-behavior generates inefficient code

The basic issue is that isel (very reasonably!) expects conditional branches
to be folded, so CGP leaving around a bunch dead computation feeding
conditional branches isn't such a good idea.  Just fold branches on constants
into unconditional branches.

llvm-svn: 123526

llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp

@@ -1071,6 +1071,9 @@ bool CodeGenPrepare::OptimizeInst(Instruction *I) {
   if (CallInst *CI = dyn_cast<CallInst>(I))
     return OptimizeCallInst(CI);
 
+  if (isa<TerminatorInst>(I))
+    return ConstantFoldTerminator(I->getParent());
+  
   return false;
 }
 

llvm/test/Transforms/CodeGenPrepare/basic.ll

@@ -4,13 +4,16 @@ target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f3
 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.
+; rdar://8785296
 define i32 @test1(i8* %ptr) nounwind ssp noredzone align 2 {
 entry:
   %0 = tail call i64 @llvm.objectsize.i64(i8* %ptr, i1 false)
   %1 = icmp ugt i64 %0, 3
   br i1 %1, label %T, label %trap
 
-; CHECK: br i1 true, label
+; CHECK: br label %T
 trap:                                             ; preds = %0, %entry
   tail call void @llvm.trap() noreturn nounwind
   unreachable