[BACKEND] add dot conversion (mma version=2) (#672)

LLVM Conversion for Dot op.

Due to the lack of `convert_layout`, currently, the dot only supports
the following combination of operands

- `$a` in shared layout
- `$b` in shared layout
- `$c` in MMA layout(but only Splat-like, leaving the generic cases to
`convert_layout`)

This PR focus on `mma.16816` related logic support, leaving the other
cases to the following PR.

Co-authored-by: Philippe Tillet <phil@openai.com>

lib/Target/LLVMIR/LLVMIRTranslation.cpp

@@ -66,7 +66,7 @@ void extractNVVMMetadata(mlir::ModuleOp module,
     // maxntid
     if (op->hasAttr(NVVMMetadataField::MaxNTid)) {
       auto attr = op->getAttr(NVVMMetadataField::MaxNTid);
-      meta.maxntidx = attr.dyn_cast<IntegerAttr>().getSInt();
+      meta.maxntidx = attr.dyn_cast<IntegerAttr>().getInt();
       hasMetadata = true;
     }