[LV] Initialize IR block pointers in ILV. (NFC) (llvm#139807)

Setting unitialized pointers to nullptr in InnerLoopVectorizer()
constructor. These were noticed during a review of the code. Seems like
a good idea to clean them up.

Author: gchaltas

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -603,13 +603,13 @@ class InnerLoopVectorizer {
   // --- Vectorization state ---
 
   /// The vector-loop preheader.
-  BasicBlock *LoopVectorPreHeader;
+  BasicBlock *LoopVectorPreHeader = nullptr;
 
   /// The scalar-loop preheader.
-  BasicBlock *LoopScalarPreHeader;
+  BasicBlock *LoopScalarPreHeader = nullptr;
 
   /// Middle Block between the vector and the scalar.
-  BasicBlock *LoopMiddleBlock;
+  BasicBlock *LoopMiddleBlock = nullptr;
 
   /// A list of all bypass blocks. The first block is the entry of the loop.
   SmallVector<BasicBlock *, 4> LoopBypassBlocks;