For original mfmaLayout, all elements for each thread are along the M
dim. Threfore, when storing results to global memory, each thread
cannot do vectorized global store since the result tensor is always
N-minor.

This PR swaps the operands of mfma instructions, and its effect is
that in the result tensor, all elements for each thread are along the
N dim. Now threads can do vectorized global store. And this can reduce
the time of the epilogue.

We already enabled swapping mfma operands for flash attention kernels
so that the results of the first dot can be kept in register and used
as the 1st operand of the second dot.

For more details about swapping operands and how it works, please
check the presentation about AMD backend at last year's triton
conference:

Bringing Triton to AMD GPUs: https://www.youtube.com/watch?v=8o7Jhbv8xek&t=1s

In general, we should use getThreadOrder in most places where getOrder
is called. Note that order and threadOrder can be different, and this
is the case for mfma.transposed layout.