[NFC] Replaced BB->getInstList().front() with BB->front()

[tmsriram]

LGTM

llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp

TEST_F(OpenMPIRBuilderTest, CanonicalLoopSimple) {
   Instruction *IndVar = Loop->getIndVar();
   EXPECT_TRUE(isa<PHINode>(IndVar));
   EXPECT_EQ(IndVar->getType(), TripCount->getType());
   EXPECT_EQ(IndVar->getParent(), Loop->getHeader());
 
   EXPECT_EQ(Loop->getTripCount(), TripCount);
 
   BasicBlock *Body = Loop->getBody();
-  Instruction *CmpInst = &Body->getInstList().front();
+  Instruction *CmpInst = &Body->front();
   EXPECT_TRUE(isa<ICmpInst>(CmpInst));
   EXPECT_EQ(CmpInst->getOperand(0), IndVar);
 
   BasicBlock *LatchPred = Loop->getLatch()->getSinglePredecessor();
   EXPECT_TRUE(llvm::all_of(successors(Body), [=](BasicBlock *SuccBB) {
     return SuccBB->getSingleSuccessor() == LatchPred;
   }));