We are running into NVPTX backend generating wrong code for an input:
```
%0 = llvm.nvvm.mma.m?n?k?.row.col.??? (...)
if laneid == 0:
ret
else:
store %0
```
The backend reorder the instruction (as an effect of `MachineSink` pass)
to
```
if laneid == 0:
ret
else:
%0 = llvm.nvvm.mma.m?n?k?.row.col.??? (...)
store %0
```
This is incorrect because `mma` is a warp instruction which needs all
threads to sync before performing the operation instead of being guarded
by a specific thread id. It should be similar as the shuffle instruction
`shfl` in terms of warp level sync, and `shfl` is marked as
`isConvergent = true`.
Apply `isConvergent = true` to `mma` instructions.
b0e9b00ce7