[NFC] Remove duplicate function calls

[dmakogon]

Summary

Removed repeated call of L->getHeader(). Now using previously stored return value.

llvm/lib/Transforms/Scalar/LoopDeletion.cpp

static bool canProveExitOnFirstIteration(Loop *L, DominatorTree &DT,
   if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
     return false;
 
   BasicBlock *Header = L->getHeader();
   // Blocks that are reachable on the 1st iteration.
   SmallPtrSet<BasicBlock *, 4> LiveBlocks;
   // Edges that are reachable on the 1st iteration.
   DenseSet<BasicBlockEdge> LiveEdges;
-  LiveBlocks.insert(L->getHeader());
+  LiveBlocks.insert(Header);
 
   SmallPtrSet<BasicBlock *, 4> Visited;
   auto MarkLiveEdge = [&](BasicBlock *From, BasicBlock *To) {
     assert(LiveBlocks.count(From) && "Must be live!");
     assert((LI.isLoopHeader(To) || !Visited.count(To)) &&
            "Only canonical backedges are allowed. Irreducible CFG?");
     assert((LiveBlocks.count(To) || !Visited.count(To)) &&
            "We already discarded this block as dead!");
static bool canProveExitOnFirstIteration(Loop *L, DominatorTree &DT,
   // 1. Traverse in topological order, so that whenever we visit a block, all
   //    its predecessors are already visited.
   // 2. If we can prove that the block may have only 1 predecessor on the 1st
   //    iteration, map all its phis onto input from this predecessor.
   // 3a. If we can prove which successor of out block is taken on the 1st
   //     iteration, mark this successor live.
   // 3b. If we cannot prove it, conservatively assume that all successors are
   //     live.
-  auto &DL = L->getHeader()->getModule()->getDataLayout();
+  auto &DL = Header->getModule()->getDataLayout();
   const SimplifyQuery SQ(DL);
   for (auto *BB : RPOT) {
     Visited.insert(BB);
 
     // This block is not reachable on the 1st iterations.
     if (!LiveBlocks.count(BB))
       continue;