For GFX10+, the MIMG instrucitons generally require a dim operand. However, when dim is missing, the assembler produces the error message "operands are not valid for this GPU or mode" (See issue https://github.com/llvm/llvm-project/issues/47585). This patch fixes the issue by producing a more direct error message. --------- Co-authored-by: Jun Wang <jun.wang7@amd.com>