Fix x86 32bits MLIR build (NFC)

This is fixing a build error:

error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'Region::iterator::difference_type' (aka 'int') in initializer list

Fix pr44767

(cherry picked from commit 31fd112)

mlir/lib/Transforms/DialectConversion.cpp

@@ -783,7 +783,7 @@ void ConversionPatternRewriterImpl::notifyRegionIsBeingInlinedBefore(
     Region &region, Region &parent, Region::iterator before) {
   for (auto &pair : llvm::enumerate(region)) {
     Block &block = pair.value();
-    unsigned position = pair.index();
+    Region::iterator::difference_type position = pair.index();
     blockActions.push_back(BlockAction::getMove(&block, {&region, position}));
   }
 }