OpenMP: Avoid using SmallVector::set_size()

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()`.

Differential Revision: https://reviews.llvm.org/D115378

Author: dexonsmith

clang/lib/CodeGen/CGStmtOpenMP.cpp

@@ -1972,7 +1972,7 @@ CodeGenFunction::EmitOMPCollapsedCanonicalLoopNest(const Stmt *S, int Depth) {
 
   // 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;

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

@@ -1831,7 +1831,7 @@ OpenMPIRBuilder::collapseLoops(DebugLoc DL, ArrayRef<CanonicalLoopInfo *> Loops,
 
   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();