[Polly] [VirtualInstruction] Avoid use of getStmtFor(BB). NFC.

[nandini12396]

Summary

With this patch, we get rid of the last use of getStmtFor(BB). Here this is done by getting the last statement of the incoming block in case the user is a phi node; otherwise just fetching the statement comprising the instruction for which the virtual use is being created.

[Meinersbur]

LGTM

polly/trunk/lib/Support/VirtualInstruction.cpp

 #include "polly/Support/SCEVValidator.h"
 
 using namespace polly;
 using namespace llvm;
 
 VirtualUse VirtualUse ::create(Scop *S, const Use &U, LoopInfo *LI,
                                bool Virtual) {
   auto *UserBB = getUseBlock(U);
-  auto *UserStmt = S->getStmtFor(UserBB);
+  Instruction *UI = dyn_cast<Instruction>(U.getUser());
+  ScopStmt *UserStmt = nullptr;
+  if (PHINode *PHI = dyn_cast<PHINode>(UI))
+    UserStmt = S->getLastStmtFor(PHI->getIncomingBlock(U));
+  else
+    UserStmt = S->getStmtFor(UI);
   auto *UserScope = LI->getLoopFor(UserBB);
   return create(S, UserStmt, UserScope, U.get(), Virtual);
 }
 
 VirtualUse VirtualUse::create(Scop *S, ScopStmt *UserStmt, Loop *UserScope,
                               Value *Val, bool Virtual) {
   assert(!isa<StoreInst>(Val) && "a StoreInst cannot be used");