[IR] Use SmallSet with more inline elements in dropUnknownNonDebugMetadata (NFC) (#98853)

SmallSet here often ends up allocating memory via std::set inside
SmallSet because KnownIDs.size() goes up to 17 on an x86 host.  This
patch switches to SmallSet<unsigned, 32> to avoid memory
allocations.

The increased inline elements here save 0.57% of heap allocations during
the compilation of X86ISelLowering.cpp.ii, a preprocessed version of
X86ISelLowering.cpp.

Author: kazutakahirata

llvm/lib/IR/Metadata.cpp

@@ -1590,7 +1590,7 @@ void Instruction::dropUnknownNonDebugMetadata(ArrayRef<unsigned> KnownIDs) {
   if (!Value::hasMetadata())
     return; // Nothing to remove!
 
-  SmallSet<unsigned, 4> KnownSet;
+  SmallSet<unsigned, 32> KnownSet;
   KnownSet.insert(KnownIDs.begin(), KnownIDs.end());
 
   // A DIAssignID attachment is debug metadata, don't drop it.