This PR decouples the operand loading from the mma codegen to make it
ready for the ongoing `DotOperandEncodingAttr` migration.
The existing DotOp conversion is composed of the following two
procedures:
1. Loading the $a,$b,$c operand from smem to registers
2. Conducting the MMA instruction codegen.
While in the latest design, the 1st stage should be part of the
`convert_layout(shared_layout) -> dot_operand_layout`, that's why the
decoupling is necessary.
Some details, this PR introduces a `MMA16816ConversionHelper` class, it
has `loadA`, `loadB` and `loadC` methods to help load $a, $b and $c from
smem to registers, both `loadA` and `loadB` methods returns a
`LLVM::Struct` which should be compatible with the new
`DotOperandEncodingAttr` conversion.
The conversion layout for $a and $b is as follows:
```c++
// The layout is a list of Value with coordinate of (i,j), the order is as
// the follows:
// [
// (0,0), (0,1), (1,0), (1,1), # i=0, j=0
// (0,2), (0,3), (1,2), (1,3), # i=0, j=1
// (0,4), (0,5), (1,4), (1,5), # i=0, j=2
// ...
// (2,0), (2,1), (3,0), (3,1), # i=1, j=0
// (2,2), (2,3), (3,2), (3,3), # i=1, j=1
// (2,4), (2,5), (2,4), (2,5), # i=1, j=2
// ...
// ]
// i \in [0, n0) and j \in [0, n1)
```
In the `convertDot` method, it takes loaded $a, $b, $c($a and $b are
type of `LLVM::Struct` while $c is a scalar Value), extract the elements
from `LLVM::Struct` following the layout above, and pass the elements to
MMA inline asm.
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