Add more clarifying comments

Author: bcardosolopes

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

@@ -1690,8 +1690,7 @@ def LLVM_BlockTagOp : LLVM_Op<"blocktag"> {
     the target address.
 
     A given function should have at most one `llvm.blocktag` operation with a
-    given `tag`. This operation cannot be used as a terminator and cannot be
-    part of an unreachable block.
+    given `tag`. This operation cannot be used as a terminator.
 
     Example:
 

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -2241,6 +2241,9 @@ static LogicalResult verifyComdat(Operation *op,
 static LogicalResult verifyBlockTags(LLVMFuncOp funcOp) {
   llvm::DenseSet<BlockTagAttr> blockTags;
   LogicalResult r = success();
+  // Note that presence of `BlockTagOp`s currently can't prevent an unrecheable
+  // block to be removed by canonicalizer's region simplify pass, which needs to
+  // be dialect aware to allow extra constraints to be described.
   funcOp.walk([&](BlockTagOp blockTagOp) {
     if (blockTags.contains(blockTagOp.getTag())) {
       blockTagOp.emitError()