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>