[mlir] Simplify code with StringMap::operator[] (NFC)#115427

kazutakahirata merged 1 commit intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_001_operator_sq_br_mlirkazutakahirata/llvm-project:cleanup_001_operator_sq_br_mlirCopy head branch name to clipboard