This PR re-lands #165873.
This PR extends the gpu.subgroup_mma_* ops to support fp64 type.
The extension requires special handling during the lowering to nvvm due
to the return type for load ops for fragment a and b (they return a
scalar instead of a struct).
The original PR did not guard the new test based on the required
architecture (sm80) which lead to a failure on the cuda runners with T4
GPUs.
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