[AMDGPU] Call isLoopExiting for blocks in the loop.

[fhahn]

Summary

isLoopExiting should only be called for blocks in the loop. A follow
up patch makes this requirement an assertion.

I've updated the usage here, to only match for actual exit blocks. Previously,
it would also match blocks not in the loop.

[nhaehnle]

LGTM

llvm/trunk/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

for (const BasicBlock *BB : L->getBlocks()) {
     for (const Instruction &I : *BB) {
       // Unroll a loop which contains an "if" statement whose condition
       // defined by a PHI belonging to the loop. This may help to eliminate
       // if region and potentially even PHI itself, saving on both divergence
       // and registers used for the PHI.
       // Add a small bonus for each of such "if" statements.
       if (const BranchInst *Br = dyn_cast<BranchInst>(&I)) {
         if (UP.Threshold < MaxBoost && Br->isConditional()) {
-          if (L->isLoopExiting(Br->getSuccessor(0)) ||
-              L->isLoopExiting(Br->getSuccessor(1)))
+          BasicBlock *Succ0 = Br->getSuccessor(0);
+          BasicBlock *Succ1 = Br->getSuccessor(1);
+          if ((L->contains(Succ0) && L->isLoopExiting(Succ0)) ||
+              (L->contains(Succ1) && L->isLoopExiting(Succ1)))
             continue;
           if (dependsOnLocalPhi(L, Br->getCondition())) {
             UP.Threshold += UnrollThresholdIf;
             LLVM_DEBUG(dbgs() << "Set unroll threshold " << UP.Threshold
                               << " for loop:\n"
                               << *L << " due to " << *Br << '\n');
             if (UP.Threshold >= MaxBoost)
               return;