[NFC] Cleanup: Replace BB->getInstList().erase() with I->eraseFromParent().

[vporpo]

Summary

This is part of a series of patches that aim at making BasicBlock::getInstList() private.

polly/lib/CodeGen/CodeGeneration.cpp

for (auto *BB : R->blocks()) {
     while (InstIt != InstEnd) {
       auto NextIt = InstIt;
       ++NextIt;
 
       if (auto *IT = dyn_cast<IntrinsicInst>(&*InstIt)) {
         switch (IT->getIntrinsicID()) {
         case Intrinsic::lifetime_start:
         case Intrinsic::lifetime_end:
-          BB->getInstList().erase(InstIt);
+          IT->eraseFromParent();
           break;
         default:
           break;
         }
       }
 
       InstIt = NextIt;
     }