[MemoryBuiltins] Use SmallDenseMap for visited map (NFC)

This partially recovers a major compile-time regression introduced
by #65326.

Author: nikic

llvm/include/llvm/Analysis/MemoryBuiltins.h

@@ -198,7 +198,7 @@ class ObjectSizeOffsetVisitor
   ObjectSizeOpts Options;
   unsigned IntTyBits;
   APInt Zero;
-  DenseMap<Instruction *, SizeOffsetType> SeenInsts;
+  SmallDenseMap<Instruction *, SizeOffsetType, 8> SeenInsts;
 
   APInt align(APInt Size, MaybeAlign Align);