OpenMP: Avoid using SmallVector::set_size()

[dexonsmith]

Summary

Update OpenMPIRBuilder::collapseLoops() to call resize() instead of
set_size(). The latter asserts on capacity limits and cannot grow,
which seems likely to be unintentional here (if it is, I think a local
assertion would be good for clarity).

Also update CodeGenFunction::EmitOMPCollapsedCanonicalLoopNest() to
use pop_back_n() instead of set_size().

[jdoerfert]

LG

[Meinersbur]

LGTM

clang/lib/CodeGen/CGStmtOpenMP.cpp

CodeGenFunction::EmitOMPCollapsedCanonicalLoopNest(const Stmt *S, int Depth) {
   EmitStmt(S);
   assert(OMPLoopNestStack.size() >= (size_t)Depth && "Found too few loops");
 
   // The last added loop is the outermost one.
   llvm::CanonicalLoopInfo *Result = OMPLoopNestStack.back();
 
   // Pop the \p Depth loops requested by the call from that stack and restore
   // the previous context.
-  OMPLoopNestStack.set_size(OMPLoopNestStack.size() - Depth);
+  OMPLoopNestStack.pop_back_n(Depth);
   ExpectedOMPLoopDepth = ParentExpectedOMPLoopDepth;
 
   return Result;
 }
 
 void CodeGenFunction::EmitOMPCanonicalLoop(const OMPCanonicalLoop *S) {
   const Stmt *SyntacticalLoop = S->getLoopStmt();
   if (!getLangOpts().OpenMPIRBuilder) {

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

OpenMPIRBuilder::collapseLoops(DebugLoc DL, ArrayRef<CanonicalLoopInfo *> Loops,
   // Build the collapsed loop body code.
   // Start with deriving the input loop induction variables from the collapsed
   // one, using a divmod scheme. To preserve the original loops' order, the
   // innermost loop use the least significant bits.
   Builder.restoreIP(Result->getBodyIP());
 
   Value *Leftover = Result->getIndVar();
   SmallVector<Value *> NewIndVars;
-  NewIndVars.set_size(NumLoops);
+  NewIndVars.resize(NumLoops);
   for (int i = NumLoops - 1; i >= 1; --i) {
     Value *OrigTripCount = Loops[i]->getTripCount();
 
     Value *NewIndVar = Builder.CreateURem(Leftover, OrigTripCount);
     NewIndVars[i] = NewIndVar;
 
     Leftover = Builder.CreateUDiv(Leftover, OrigTripCount);
   }