[Backport to 16] Fix uninitialized DominatorTree member (#2734)

(cherry picked from commit a4010e0)

lib/SPIRV/SPIRVWriter.cpp

@@ -5063,7 +5063,7 @@ static auto stablePreDominatorTraversal(Function &F, const DominatorTree &DT) {
                                     std::forward_iterator_tag, BasicBlock> {
 
     // The passed DominatorTree; may be unset for end iterators.
-    const DominatorTree *DT;
+    const DominatorTree *DT = nullptr;
 
     // The set of basic blocks already visited in this traversal.
     SmallPtrSet<const BasicBlock *, 4> VisitedBBs;